Fix async polymorphic belongsTo serialization

Closes #2551

Very naive implementation, also I didn't change anything in the embedded mixin because I don't know how/if it's supposed to be handled

I think this should be considered as a breaking change because even if the signature does not change, the passed record to `serializePolymorphicType` is the inverse record instead of the record itself.

cc/ @jherdman @igorT