Fixed: Firefox needs the XHR to be async when using withCredentials=true

So here I force the POST request to have the same ``async`` param than the GET.
And I force ``withCredentials=false`` on closing.

It works without modification for the jQuery version because you always make async requests:
l.1712 ``ajaxRequest.open(request.method, url, true);``

Thanks,
Sylvain.