Mockjax not working when contentType is specified in the request

Question

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 null when the request contentType is modified.

The working JS 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();
    });
});

However, if I set the content type in the $.ajax request thusly:

    $.ajax({
        url: '/test',
        type: 'POST',
        contentType: 'application/json',
    })

The test fails:

Expected:   
{
    "foo": "bar"
}
Result:     
    null

I have a jsfiddle here which demonstrates the the problem.

What am I doing wrong? Or is this a bug, or incompatibility between JS libraries?

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.

Note: 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.

Answer 1

Adding dataType: 'json' to the $.ajax call helps:

$.ajax({
    url: '/test',
    type: 'POST',
    dataType: 'json',
    contentType: 'application/json',
})

... but I have no idea why...