promise reject 一直等待响应
```javascript
var Koa = require( 'koa2' );
var http = require( 'http' );

var webserv = new Koa();
webserv.use( async ( ctx, next ) => {
	// 执行这一条代码会一直在等待
	await new Promise( ( resolve, reject ) => setTimeout( reject, 100 ) );
	// 执行这一条会正常显示Not Found
	await new Promise( ( resolve, reject ) => setTimeout( reject, 100 ) ).then( () => console.log( 'resolve' ), () => console.log( 'reject' ) );
} );
var httpserv = http.createServer();
httpserv.on( 'request', webserv.callback() );
httpserv.listen( 19000 );
```