Better support both HTTP Accept mimetypes and ?api_format= for resources.

Our code hasn't really worked properly with ?api_format= for a while, mostly
due to supporting mimetypes listed in HTTP Accept. We now support both,
being careful to use the api_format when passed, and falling back on the
mimetype.

Did some manual testing using curl and Firefox to make sure I saw the results I expected. Also used this in my webapi resources change.