mounting into an express app
I'm exploring migrating a large production Express app to koa v2 (so all of this applies to the promise API of v2). The koajs.com docs say for `app.callback()`: 

> "You may also use this callback function to mount your koa app in a Connect/Express app." 

However, it seems like you must mount a koa app at the "bottom" of your Express app since koa will never call `next()` to yield control back to express (like in the 404 case). I can't see an obvious work around for this currently. If the function returned by `callback` returned the promise, I think something like the following would work:

``` js
const koaCallback = koaApp.callback();
expressApp.use(function (req, res, next) {
  koaCallback(req, res)
    .then(function () {
      // not sure exactly how to detect this but seems doable
      if (!koaSentResponse(res)) {
        next() // let express keep processing this request
      }
   });
});
```

Perhaps somewhat related -- what's the use case for setting `ctx.respond` to `false`? 
