BodyListener format fallback

As described in https://github.com/FriendsOfSymfony/FOSRestBundle/issues/495, if you don't set Content-type BodyListener will not fallback to `%fos_rest.routing.loader.default_format%`. Reason why need to fallback is because it will return response in format set by `%fos_rest.routing.loader.default_format%` so input conversion should be desired behavior also.