[MOD] Use server_base instead of the API host

This wasn't an issue because the server I test on has the same API host
as its name.

src/Routes/UserAck.c

@@ -97,11 +97,8 @@ RouteHead(RouteRoomAck, arr, argp)
         );
         goto end;
     }
-    creator = StrConcat(
-        4, 
-        "@", args->data->config->sender_localpart, ":", 
-        args->data->config->homeserver_host
-    );
+    creator = ParseeMXID(args->data);
+            
     id = ASCreateRoom(
         args->data->config, 
         creator, room