jQuery.AJAX uses GET instead of POST
<p>I have a really simple HTML page running jQuery that's trying to post to a REST API. Here's my code.</p>

<pre><code>&lt;script type="text/javascript" language="javascript"&gt;
    var user = 'someuser';
    var pass = 'somepassword';
    var payload = {
        "value1": "first value",
        "value2": "second value"
    };
    var rootUrl = 'http://someinternalserver:8888/api/Method';
&lt;/script&gt;
&lt;script language="javascript" type="text/javascript" id="postWtnBlock"&gt;
    function postValue_Go() {
        $.ajax({
            url: rootUrl,
            // Removing the next line or changing the value to 'JSON' results in an OPTIONS request.
            dataType: 'JSONP',
            data: JSON.stringify(payload),
            method: 'POST',
            user: user,
            password: pass,
            beforeSend: function (req) {
                req.setRequestHeader('Authorization', 'BASIC ' + btoa(user + ':' + pass));
            },
            success: function (data) {
                alert(data);
            },
            error: function (xhr) {
                alert(xhr.status + ' ' + xhr.statusText);
            }
        });
    }
&lt;/script&gt;
</code></pre>

<p>Two things are happening here.</p>

<p>1) Every request is being sent as a GET request, not a POST.
2) The Authorization Header never makes it into the request; it's always missing.</p>

<p>I have no idea why this is.</p>

<p><strong>Update #1:</strong> The new postValue_Go() looks like this...</p>

<pre><code>    function postValue_Go() {
        $.ajax({
            url: rootUrl,
            data: JSON.stringify(payload),
            method: 'POST',
            username: user,
            password: pass,
            xhrFields: {
                withCredentials: true
            },
            success: function (data) {
                alert(data);
            },
            error: function (xhr) {
                alert(xhr.status + ' ' + xhr.statusText);
            }
        });
    }
</code></pre>

<p><em>Here's the raw request, captured in Fiddler:</em></p>

<pre><code>POST http://someinternalserver:8888/api/Method/ HTTP/1.1
Host: someinternalserver:8888
Connection: keep-alive
Content-Length: 693
Accept: */*
Origin: http://devserver
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: http://devserver/samples/ExternalAPI/
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.8
Cookie: someCookieValue=Boo; someOtherCookieValue=Yum;

{"value1": "first value","value2": "second value"}
</code></pre>

<p>And the raw response, that too captured in Fiddler.</p>

<pre><code>HTTP/1.1 401 Unauthorized
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/7.5
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Fri, 19 Aug 2016 17:12:29 GMT
Content-Length: 61

{"Message":"Authorization has been denied for this request."}
</code></pre>