bring mg_strdup() back

tutorials/core/multi-threaded/main.c

@@ -50,8 +50,7 @@ static void fn(struct mg_connection *c, int ev, void *ev_data) {
       // Multithreading code path
       struct thread_data *data =
           (struct thread_data *) calloc(1, sizeof(*data));  // Worker owns it
-      data->message.buf = mg_mprintf("%.*s", hm->message.len, hm->message.buf);
-      data->message.len = hm->message.len;  // Pass message
+      data->message = mg_strdup(hm->message); // Pass message
       data->conn_id = c->id;
       data->mgr = c->mgr;
       start_thread(thread_function, data);  // Start thread and pass data