Fix resource_name support for ResourceRelatedField

Author: czosel

rest_framework_json_api/relations.py

@@ -145,7 +145,7 @@ def to_representation(self, value):
         # check to see if this resource has a different resource_name when
         # included and use that name
         resource_type = None
-        root = getattr(self.parent, 'parent', self.parent)
+        root = getattr(self.parent, 'parent', self.parent) or self.parent
         field_name = self.field_name if self.field_name else self.parent.field_name
         if getattr(root, 'included_serializers', None) is not None:
             includes = get_included_serializers(root)