I can't use settimeout in middle ware?
```
const Koa = require('koa')
var cors = require('koa2-cors')

const app = new Koa()
app.use(cors())

app.use(async ctx => {
    //ctx.body = "it can be sent successfully"
    global.setTimeout(()=>{
        ctx.body ="it will falure"
    },1000*.1)
})

app.listen(9999)
```

If I use `setTimeout` in function, it will result in a 404 in browser, why?