fix: Update LocaleData plugin to support dayjs().longDateFormat()
fix #680
What is the equivalent of this moment code?

`moment.localeData().longDateFormat('L')`
How to get the locale format?
diff --git a/src/plugin/localeData/index.js b/src/plugin/localeData/index.js
index c8008a22d..329090d24 100644
--- a/src/plugin/localeData/index.js
+++ b/src/plugin/localeData/index.js
@@ -17,7 +17,8 @@ export default (o, c, dayjs) => { // locale needed later
       weekdaysMin: instance =>
         (instance ? instance.format('dd') : getShort(this, 'weekdaysMin', 'weekdays', 2)),
       weekdaysShort: instance =>
-        (instance ? instance.format('ddd') : getShort(this, 'weekdaysShort', 'weekdays', 3))
+        (instance ? instance.format('ddd') : getShort(this, 'weekdaysShort', 'weekdays', 3)),
+      longDateFormat: format => this.$locale().formats[format]
     }
   }
   proto.localeData = function () {

diff --git a/test/plugin/localeData.test.js b/test/plugin/localeData.test.js
index 151713b67..289832873 100644
--- a/test/plugin/localeData.test.js
+++ b/test/plugin/localeData.test.js
@@ -2,8 +2,10 @@ import MockDate from 'mockdate'
 import moment from 'moment'
 import dayjs from '../../src'
 import localeData from '../../src/plugin/localeData'
+import localizedFormat from '../../src/plugin/localizedFormat'
 import '../../src/locale/zh-cn'
 
+dayjs.extend(localizedFormat)
 dayjs.extend(localeData)
 
 beforeEach(() => {
@@ -14,7 +16,7 @@ afterEach(() => {
   MockDate.reset()
 })
 
-it('instance localeData', () => {
+it('Instance localeData', () => {
   const d = dayjs()
   const m = moment()
   const dayjsLocaleData = dayjs().localeData()
@@ -28,9 +30,14 @@ it('instance localeData', () => {
   expect(dayjsLocaleData.weekdaysMin()).toEqual(momentLocaleData.weekdaysMin())
   expect(dayjsLocaleData.weekdaysShort(d)).toBe(momentLocaleData.weekdaysShort(m))
   expect(dayjsLocaleData.weekdaysShort()).toEqual(momentLocaleData.weekdaysShort())
+  const longDateFormats = ['LT', 'LTS', 'L', 'LL', 'LLL', 'LLLL']
+  longDateFormats.forEach((f) => {
+    expect(dayjsLocaleData.longDateFormat(f)).toEqual(momentLocaleData.longDateFormat(f))
+  })
 })
 
-it('global localeData', () => {
+
+it('Global localeData', () => {
   dayjs.locale('zh-cn')
   moment.locale('zh-cn')
   let dayjsLocaleData = dayjs.localeData()

{"test/plugin/localeData.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"test/timezone.test.js:Add Time days (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/get-set.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/quarterOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/advancedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toArray.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrBefore.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Utc Offset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Diff (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc-utcOffset.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/keys.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/utils.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/relativeTime.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/parse.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isMoment.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toObject.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/localizedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/constructor.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrAfter.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isLeapYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/comparison.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/query.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/calendar.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/declarations.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/badMutable.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/customParseFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/ru.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/minMax.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/dayOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeeksInYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC add day in DST": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/manipulate.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/buddhistEra.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isBetween.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}}
{"test/plugin/localeData.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"passed_count": 41, "failed_count": 1, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/plugin.test.js", "test/get-set.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/plugin/isSameOrBefore.test.js", "test/timezone.test.js:Utc Offset", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/plugin/isLeapYear.test.js", "test/comparison.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/plugin/calendar.test.js", "test/plugin/declarations.test.js", "test/locale.test.js", "test/plugin/badMutable.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/plugin/buddhistEra.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/display.test.js"], "skipped_tests": []}
{"passed_count": 40, "failed_count": 2, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/plugin.test.js", "test/get-set.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/comparison.test.js", "test/plugin/isLeapYear.test.js", "test/query.test.js", "test/plugin/calendar.test.js", "test/plugin/declarations.test.js", "test/locale.test.js", "test/plugin/badMutable.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/plugin/buddhistEra.test.js", "test/manipulate.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/plugin/localeData.test.js", "test/display.test.js"], "skipped_tests": []}
{"passed_count": 41, "failed_count": 1, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/get-set.test.js", "test/plugin.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/weekYear.test.js", "test/plugin/toArray.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/plugin/isLeapYear.test.js", "test/comparison.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/plugin/calendar.test.js", "test/plugin/declarations.test.js", "test/locale.test.js", "test/plugin/badMutable.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/minMax.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/plugin/buddhistEra.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/display.test.js"], "skipped_tests": []}