Why do I have to call 'return' on ctx.render
Why does this works:
```
app.use((ctx, next) => {
  return ctx.render('user', { user }); // return is important, I don't know why
});
```
and this doesn't:
```
app.use((ctx, next) => {
  ctx.render('user', { user }); // Response 404
});
```