Favicon

#701 is really annoying. Favicons are trouble, always.

I've added some tests and it's (obviously) doing what it says. I thought it might be needing to use a less explicit path maybe using `asset_path`, but that just gave me nothing useful at all `/favicon/01.png` instead of `/assets/favicon/01.png` rasserfrassin'.

Anybody got any ideas? It didn't work for a bit but now is working properly on my local machine but the code has ended up the same as it started.