temporarily assign the client a nick during reattach

(So that the registration burst displays correctly)

irc/client_lookup_set.go

@@ -160,7 +160,9 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 		if !currentClient.AddSession(session) {
 			return errNicknameInUse
 		}
-		// successful reattach:
+		// successful reattach. temporarily assign them the nick they'll have going forward
+		// (the current `client` will be discarded at the end of command execution)
+		client.updateNick(currentClient.Nick(), newcfnick, newSkeleton)
 		return nil
 	}
 	// analogous checks for skeletons