Attempt two different encodings for request body

[jakeanderson1]

Hello, I recently have been starting to use apistar somewhat and came across the TODO related to generically supporting request body media types generically and I made it slightly more generic than it was but admittedly haven't solved issue #669 fully. This seems to allow me to work around the media type and POST with the "multipart/form-data" encoding so I wanted to submit a PR in case this would be useful to others, but if it would be helpful for me to continue and support more media types as well I'd be happy to do so.