unwrap FormData in XMLHttpRequest.send if needed

fixes https://github.com/Polymer/polymer/issues/725

So, I kind of liked this one. XHR.send takes a bunch of things most of which are not wrapped, so I can see someone looking at this code and thinking "why are we patching XHR?". Maybe I'll mention FormData in the comment above. My thinking was that we didn't think of the XHR<->FormData connection when patching FormData, so someone looking at this file might not grasp it immediately either.