message: modify return value format of client list
diff: @@ -430,14 +430,14 @@ void ClientCmd::Do() {
  std::vector<ClientInfo> clients;
  g_pika_server->ClientList(&clients);
  std::vector<ClientInfo>::iterator iter= clients.begin();
  - std::string reply = "+";
  + std::string reply = "";
  char buf[128];
  while (iter != clients.end()) {
  snprintf(buf, sizeof(buf), "addr=%s fd=%d idle=%ld\n", iter->ip_port.c_str(), iter->fd, iter->last_interaction == 0 ? 0 : now.tv_sec - iter->last_interaction);
  reply.append(buf);
  iter++;
  }
  - res_.AppendContent(reply);
  + res_.AppendString(reply);
  } else if (operation_ == CLIENT_KILL_S && ip_port_ == "all") {
  g_pika_server->ClientKillAll();
  res_.SetRes(CmdRes::kOk);
  