jsonapi-serializers: serialize_errors support claims for activelmodel are wrong

From https://github.com/fotinakis/jsonapi-serializers/blob/master/lib/jsonapi-serializers/serializer.rb#L340-L356

The problem is, the lib assumes that the errors hash is an activemodel hash if it responds to #to_hash and #full_messages. First there’s nowhere in the lint spec from activel model regarding these methods, so it’s a false assumption. Second, many objects implement #to_hash without arguments, and this gem assumes one hash argument with support for :full_messages key, which is also not part of the spec.

These assumptions make this gem fail for the sequel ORM, which is ActiveModel-compliant, has an errors object, but this doesn’t support #to_hash with arguments.

I think the lib should add suport explicitly for active record error hashes and fallback to something else.