How to query jQuery AJAX?

The following two ways to implement ajaxRequest (1) (2) should be equivalent.
Having said that:

• Why is unit test (3) executed, which checks the callback, succeeds in (1) and doesn't work (2)?
• How can I rewrite test (3) to track a successful response in (2)?
• If I try to stub jQuery.ajaxuse sinon and code (2), I get an error. How to fix it?

See comments in code (3) for more details.

(1)

ajaxRequest: function (message, callback) {
    return $.ajax({
        url: backendRouter.generate('feedback_send'),
        type: 'POST',
        dataType: 'json',
        data: {
            message: message
        },
        success: callback
    });
}

(2)

ajaxRequest: function (message, callback) {
    return $.ajax({
        url: backendRouter.generate('feedback_send'),
        type: 'POST',
        dataType: 'json',
        data: {
            message: message
        }
    }).success(callback);
}

(3)

it("should execute the callback function on success", function () {
    spyOn($, "ajax").andCallFake(function(options) {
        options.success();
    }); // If I use the code (2) I get the following error
        // TypeError: Object #<Object> has no method 'success'
    var callback = jasmine.createSpy();
    ajaxRequest('some message', callback);
    expect(callback).toHaveBeenCalled();
});

(4)

it("makes a GET request for todo items", function () {
    sinon.stub(jQuery, 'ajax');
    backendController.ajaxRequest('some message', sinon.spy()); 
    // Cannot call method 'success' of undefined
});