fix: Duration plugin supports parse ISO string with week (W)
fix #949
**Describe the bug**
When creating duration object by parsing ISO 8601 string which include week designator (`W`), the returned duration object does not take into account the week information.

For example:
```javascript
dayjs.duration('P2M3W4D').toISOString();   // "P2M4D"
dayjs.duration('P2M3W4D').asDays();        // 64
dayjs.duration('P2M3W4D').asWeeks();       // 9.142857142857142
```

For reference, the following is the same operation using moment.js v2.27.0
```javascript
moment.duration('P2M3W4D').toISOString();  // "P2M25D"
moment.duration('P2M3W4D').asDays();       // 86
moment.duration('P2M3W4D').asWeeks();      // 12.285714285714286

// FWIW: moment.js use 2 months = 61 days... but that's whole nother issue
// - https://github.com/moment/moment/issues/2688
// - https://github.com/moment/moment/issues/5603
```

**Expected behavior**
When parsing ISO 8601 string duration, the week designator should also correctly set the weeks information.

**Information**
 - Day.js v1.8.28
 - OS: Ubuntu 18.04.4 LTS
 - Browser: Chrome 85.0.4181.8 (Official Build) dev (64-bit)
 - Time zone: GMT+07:00

Day.js returns wrong duration object when created using ISO 8601 duration with WEEK designator (W)
diff --git a/src/plugin/duration/index.js b/src/plugin/duration/index.js
index 28031b127..31ab68572 100644
--- a/src/plugin/duration/index.js
+++ b/src/plugin/duration/index.js
@@ -48,7 +48,7 @@ class Duration {
       const d = input.match(durationRegex)
       if (d) {
         [,,
-          this.$d.years, this.$d.months,,
+          this.$d.years, this.$d.months, this.$d.weeks,
           this.$d.days, this.$d.hours, this.$d.minutes, this.$d.seconds] = d
         this.calMilliseconds()
         return this
@@ -83,7 +83,7 @@ class Duration {
   toISOString() {
     const Y = this.$d.years ? `${this.$d.years}Y` : ''
     const M = this.$d.months ? `${this.$d.months}M` : ''
-    let days = this.$d.days || 0
+    let days = +this.$d.days || 0
     if (this.$d.weeks) {
       days += this.$d.weeks * 7
     }

diff --git a/test/plugin/duration.test.js b/test/plugin/duration.test.js
index e854d9148..d2e4abe6c 100644
--- a/test/plugin/duration.test.js
+++ b/test/plugin/duration.test.js
@@ -62,6 +62,12 @@ describe('Parse ISO string', () => {
   it('Part ISO string', () => {
     expect(dayjs.duration('PT2777H46M40S').toISOString()).toBe('PT2777H46M40S')
   })
+  it('ISO string with week', () => {
+    const d = dayjs.duration('P2M3W4D')
+    expect(d.toISOString()).toBe('P2M25D')
+    expect(d.asDays()).toBe(85) // moment 85, count 2M as 61 days
+    expect(d.asWeeks()).toBe(12.142857142857142) // moment 12.285714285714286
+  })
   it('Invalid ISO string', () => {
     expect(dayjs.duration('Invalid').toISOString()).toBe('P0D')
   })

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