message: Fix ping command
  The ping command checked for the wrong number of arguments before... causing ardb to crash whenever pinged. See expected behavior:
diff: @@ -37,7 +37,7 @@ OP_NAMESPACE_BEGIN
  }
  int Ardb::Ping(Context& ctx, RedisCommandFrame& cmd)
  {
  - if(cmd.GetArguments().size() == 1)
  + if(cmd.GetArguments().size() == 0)
  {
  ctx.GetReply().SetStatusCode(STATUS_PONG);
  }
  