this.body setter does not respect this.respond
If I explicitly set this.respond to false in my handler like this:

``` javascript
app.use(function * () {
  this.respond = false
  this.res.end()
  this.body = 'foo'
})
```

If anything sets the body after the response was ended, I get the following error:

> Error: Can't remove headers after they are sent.

https://github.com/koajs/koa/blob/master/lib/response.js#L174 the error maybe on this line.
