Allow URLSearchParams to be passed to XHR Send()

This also updates us to the new webidl that uses FetchBodyInit (from the fetch spec)

I haven't done FormData since we don't have file support (so it would only work for String FormDatums), but the [algorithm](http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#multipart/form-data-encoding-algorithm) isn't too complicated.

Blocks #2282