Move arg default to the start of the function

Also don't overwrite the list that gets passed in.

synapse/handlers/room.py

@@ -493,6 +493,8 @@ class RoomMemberHandler(BaseHandler):
         Raises:
             SynapseError if there was a problem changing the membership.
         """
+        remote_room_hosts = remote_room_hosts or []
+
         target_user = UserID.from_string(event.state_key)
         room_id = event.room_id
 
@@ -537,8 +539,7 @@ class RoomMemberHandler(BaseHandler):
                     pass
                 else:
                     # send the rejection to the inviter's HS.
-                    remote_room_hosts = remote_room_hosts or []
-                    remote_room_hosts.append(inviter.domain)
+                    remote_room_hosts = remote_room_hosts + [inviter.doman]
                     action = "remote_reject"
 
         federation_handler = self.hs.get_handlers().federation_handler