Single record mode and root false tweaks

Fallback to ruby serializer for root: false option (until we add support for it)
Implement single_record for ruby array serializer (to unify behavior when mixing ruby/db serializers)
Auto-singularize root key in single record mode (specifying root: 'singular_name' is now optional)

There currently are no tests for the fallback and single_record behavior, but we should add it after #53 is implemented.

DavyJonesLocker / postgres_ext-serializers