https://github.com/koajs/koa/blob/master/lib/application.js#L188 will set a string body, and will end in https://github.com/koajs/koa/blob/master/lib/application.js#L198, won't set a length.
so it will block the request in browser if:

```
this.body = null;
this.status = 200;
```

all these things are such a mess, seems need to make sure: if body not null, need make sure set length, i'll check my code, fix what i can find.