This looks like a sensible change, but it may want a unit test.  I do notice that there's no unit testing for sendAjax at all, and I'm assuming it's an public API.  However, that might not be true.  Is it?  If not, I'd suggest adding an XXX comment clarifying that it's internal and suggesting renaming it to _sendAjax at some point.

If it does need to be an public API, sendAjax might well get enough coverage from the stuff that already uses the fake XHR.  

Thoughts?