Easily send a 201 response with no body
I would like to be able to:

* Set `ctx.status` to `201`
* Set `ctx.body` to `''`, and
* Exit the middleware function

In a single statement.

In Hapi I could do this: `return reply('').code(201);`

Currently in Koa I am doing it like this:

```
ctx.status = 200;
ctx.body = '';
return;
```

I'd love to be able to do something like:

`return ctx.setResponse(201, '')`

Is this something that has been considered? Have I just missed something obvious? :)