order 字段名添加分隔符

order方法中的字段名 rank 不会自动添加 `` ,导致在某些MySQL版本中会报错

application/index/model/CronHandler.php

@@ -17,7 +17,7 @@ class CronHandler extends Model{
 	public $notifiedUid = [];
 
 	public function __construct(){
-		$this->cornTasks = Db::name('corn')->where('enable',1)->order('rank desc')->select();
+		$this->cornTasks = Db::name('corn')->where('enable',1)->order('`rank` desc')->select();
 		$this->timeNow = time();
 	}