Added an option to normalize other content types

The normalizer is only applied when the data is decoded which is not the case with `multipart/form-data`.

Fix issue https://github.com/FriendsOfSymfony/FOSRestBundle/issues/868.
Indeed .. these two seem to be the only ones:
https://twitter.com/lsmith/status/542616104972333057

so is there a use case where I only want it for one mime type but not for the other?
imho it would be better to just have a flag for this ..