Returning HTML on Fastboot success prevents option of manipulating HTML afterFastboot

My use case is that I want to cache the HTML that is generated from Fastboot. I use fastboot-express-middleware as middleware with fastboot-app-server. This has an afterFastboot hook, which is only accessible on an error because `index.js line 51` `res.send(html)` just returns the HTML rather than calling `next()`.

What I currently do is replace that line with `next()` and pass through the html on the response object.

Is this something that might be good to change, and how would it affect users not using fastboot-app-server?

Happy to make a pull request if you did think this was a valid change.