Prefer XMLHttpRequest.

There is no reason to give up XMLHttpRequest when it is available. This pull change also makes it possible to use a more complex XMLHttpRequest shim with SuperAgent.

Ref:
https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started
http://www.quirksmode.org/js/xmlhttp.html
https://github.com/jquery/jquery/blob/1.x-master/src/ajax/xhr.js#L24