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` yet since we don't have file support (so it would only work for `DOMString` `FormDatum`s), 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