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.
The fetch default is `self.class._root`, which could not be set. It's a 3-step default: root option => serializer root => each serializer root. My change actually occurs in the last case, when the serializer root isn't set.