[BUG] Fix Serialization of Async belongsTo

belongsTo associations are promises, and their constructor were not being
identified correctly when piecing together its type.

This change fixes this issue in both the REST Serializer, and AMS
Serializer first by ensuring the belongsTo association is resolved. I've
also refactored the two serializers to share more common knowledge about
this activity.
WIP
WIP -- re: feedback