title: Mockjax not working when contentType is specified in the request
body: <p>I am attempting to test some AJAX-related code with QUnit and jQuery Mockjax, and have run into a problem that Mockjax always appears to return <code>null</code> when the request contentType is modified.</p>
  
  <p>The working JS code:</p>
  
  <pre><code>QUnit.asyncTest('$.ajax POST', function() {
      expect(1);
      $.mockjax({
          url: '/test',
          type: 'POST',
          responseText: {
              foo: 'bar'
          }
      });
      $.ajax({
          url: '/test',
          type: 'POST',
      })
      .done( function(data,textStatus,jqXHR) {
          deepEqual(data,{foo:'bar'},'Correct response');
          QUnit.start();
      });
  });
  </code></pre>
  
  <p>However, if I set the content type in the <code>$.ajax</code> request thusly:</p>
  
  <pre><code>    $.ajax({
          url: '/test',
          type: 'POST',
          contentType: 'application/json',
      })
  </code></pre>
  
  <p>The test fails:</p>
  
  <pre><code>Expected:   
  {
      "foo": "bar"
  }
  Result:     
      null
  </code></pre>
  
  <p>I have a <a href="http://jsfiddle.net/mJxDU/1/" rel="nofollow">jsfiddle here</a> which demonstrates the the problem.</p>
  
  <p>What am I doing wrong? Or is this a bug, or incompatibility between JS libraries?</p>
  
  <p>I am using jquery 1.7.2, mockjax 1.5.3, and QUnit 1.14.0. The same behavior is observed in Chrome and Firefox.</p>
  
  <p><i><strong>Note:</strong> Naturally in my production code I will be sending JSON-encoded data in the request as well, but for the purpose of demonstration, I have reduced the problem to what I believe is the minimum necessary to reproduce my problem.</i></p>