Use result of serializer parameter encoding rather than re-encoding as form data for multipart requests

As pointed out in #1850, there's a mismatch of expectations when a JSON serializer encodes parameters as form data in multipart requests. This patch changes this behavior to use the www-form-url-encoded query string / JSON / plist / etc. body and `Content-Type` as a form-data part rather than attempting to build it manually.

I don't want to merge this in until I can be reasonably sure that it doesn't completely break existing behavior, so I'd appreciate anyone who could give this a try and report back.