Added an option to normalize form data

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.
Wouldn't it be better to use `replaceArgument()` instead (makes it less error prone when at some point in the future the definition is modified somewhere else in the extension class or something like that).