message: alter the return result while query
diff: * Copyright 2021 gStore, All Rights Reserved.
  * @Author: Bookug Lober suxunbin liwenjie
  * @Date: 2021-08-22 00:37:57
  - * @LastEditTime: 2021-09-02 20:26:35
  + * @LastEditTime: 2021-09-03 14:34:43
  * @LastEditors: Please set LastEditors
  * @Description: the http server to handler the user's request, which is main access entrance of gStore
  * @FilePath: /gstore/Main/ghttp.cpp
  @@ -2578,18 +2578,21 @@ string update_flag,string remote_ip,string thread_id,string log_prefix)
  StringBuffer resBuffer;
  PrettyWriter<StringBuffer> resWriter(resBuffer);
  resDoc.Accept(resWriter);
  - success = resBuffer.GetString();
  + string resJson = resBuffer.GetString();
  localname = localname + "." + format;
  filename = filename + "." + format;
  filename = "";
  filename = "sparql." + format;
  cout << log_prefix << "filename: " << filename << endl;
  - *response << "HTTP/1.1 200 OK\r\nContent-Length: " << success.length();
  - *response << "\r\nContent-Type: application/octet-stream";
  - *response << "\r\nContent-Disposition: attachment; filename=\"" << filename << '"';
  - *response << "\r\n\r\n" << success;
  + *response << "HTTP/1.1 200 OK\r\nContent-Type: application/json\r\nContent-Length: " << resJson.length();
  + *response << "\r\nCache-Control: no-cache"
  + << "\r\nPragma: no-cache"
  + << "\r\nExpires: 0";
  + *response << "\r\n\r\n" << resJson;
  + cout<<"query complete! unlock the database "<<endl;
  pthread_rwlock_unlock(&(it_already_build->second->db_lock));
  + cout<<"query complete! unlock the database successfully! "<<endl;
  return;
  //return true;
  //
  