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.
actually maybe we should replace the check for something like:
```
if ((!in_array($contentType, array('multipart/form-data', 'application/x-www-form-urlencoded'), true) || $method != 'POST')
  && in_array($method, array('POST', 'PUT', 'PATCH', 'DELETE'))
) {
```