Get relationship id from the resource

We should avoid getting relationship id from the `model` directly to allow clients to customize how relationship ids are fetched. The resource exposes the relationship id through a method which delegates to the `model`. This way we allow clients to override the relationship id in the resource to fit their needs.

Author: gottfrois

lib/jsonapi/resource_serializer.rb

@@ -427,11 +427,11 @@ def link_object(source, relationship, include_linkage = false)
     def foreign_key_value(source, relationship)
       related_resource_id = if source.preloaded_fragments.has_key?(format_key(relationship.name))
         source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
-      elsif source._model.respond_to?("#{relationship.name}_id")
+      elsif source.respond_to?("#{relationship.name}_id")
         # If you have direct access to the underlying id, you don't have to load the relationship
         # which can save quite a lot of time when loading a lot of data.
         # This does not apply to e.g. has_one :through relationships.
-        source._model.public_send("#{relationship.name}_id")
+        source.public_send("#{relationship.name}_id")
       else
         source.public_send(relationship.name).try(:id)
       end