Alex Tatarnikov

Hi, I've noticed that [`#validate_includes!`](https://github.com/Netflix/fast_jsonapi/blob/master/lib/fast_jsonapi/object_serializer.rb#L299) validates only first element of the `include` option array. The reason for this behavior is [`#detect`](https://github.com/Netflix/fast_jsonapi/blob/master/lib/fast_jsonapi/object_serializer.rb#L302) method that was used in order to iterate through...