Return undefined in the route method to tell that you ended the response yourself

See http://stackoverflow.com/q/17073431/220060 for a use case.

We had a similar use case. We need to stream a file with seek to implement pseudostreaming.

I also updated the documentation. Our team got very confused when they discovered that they could return a text instead of a template name. I read the source code and documented what's possible. Of course I added the new possibility to return undefined.