Use the proper serialization format when bundling aggregations. (#12090)

This ensures that the `latest_event` field of the bundled aggregation
for threads uses the same format as the other events in the response.

tests/rest/client/test_relations.py

@@ -704,10 +704,8 @@ class RelationsTestCase(BaseRelationsTestCase):
                         }
                     },
                     "event_id": thread_2,
-                    "room_id": self.room,
                     "sender": self.user_id,
                     "type": "m.room.test",
-                    "user_id": self.user_id,
                 },
                 relations_dict[RelationTypes.THREAD].get("latest_event"),
             )