fix escape function call

[redybai]

src/Server.php的line519原来为 $this->db->escape(json_encode($msg));
client发消息后 ，server报错：Fatal error: Uncaught Error: Call to undefined method Swoole\Coroutine\MySQL::escape() in webim/src/Pool.php:48

改为 $_msg = Filter::escape(json_encode($msg));后程序执行正常。
烦请看一下修改是否正确。