Lookup polymorphic type from the column before the record (#1326)

Currently, `Relationship#type_for_source` looks up polymorphic source's
type by:

1. Fetching its associated record.
2. Calling `.class._type` on that record.

This approach is inefficient because usually, a polymorphic record (the
`source`) has
`_type` and `_id` columns. And the `_type` column's value will be the
polymorphic relationship record's type.

So this PR makes `type_for_source` check the type value from column
before fetching the associated record.

lib/jsonapi/relationship.rb

@@ -55,8 +55,14 @@ def relation_name(options)
 
     def type_for_source(source)
       if polymorphic?
-        resource = source.public_send(name)
-        resource.class._type if resource
+        # try polymorphic type column before asking it from the resource record
+        if source._model.respond_to?(polymorphic_type)
+          model_type = source._model.send(polymorphic_type)
+          source.class.resource_for(model_type)._type if model_type
+        else
+          resource = source.public_send(name)
+          resource.class._type if resource
+        end
       else
         type
       end