Allow ArraySerializer to inherit :each_serializer root

**This PR replaces #619 as there was too many conflicts after multiple rebases.**

This change make ArraySerializer inherit `each_serializer`'s root when no specific root is set.

I'ts just a way to avoid definig root twice and keep things DRY.

JSON API now requires the same (plural) root for single resource and collections.
I don't understand what circumstances this would happen. If you fetch it and it doesn't exist, you set it already.