use process.nextTick as middleware, response 404
```
const Koa = require('koa');
const app = new Koa();

const {promisify} = require('util');

const sleep = promisify(setTimeout);

app.use((ctx, next) => {
    process.nextTick(() => {
        next();
    });
});


app.use(async ctx => {
    await sleep(2000);
    ctx.body = 'Hello World';
});

app.listen(3000);
```

when run above this code.  invoke command `curl localhost:3000 -v `. show 404 statusCode. 