Responder :

- when an error occurred, returns unprocessable_entity (422)
- when PUT, return the resource with ok (200)

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Fix https://github.com/ccocchi/rabl-rails/issues/34

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>