Don't corrupt cache

synapse/groups/groups_server.py

@@ -109,6 +109,7 @@ class GroupsServerHandler(object):
                 room_id, len(joined_users),
                 with_alias=False, allow_private=True,
             )
+            entry = dict(entry)  # so we don't change whats cached
             entry.pop("room_id", None)
 
             room_entry["profile"] = entry