Added HEAD to RouterGroup.Static

Hi!

I use the [live.js browser extension](https://chrome.google.com/webstore/detail/livepage/pilnojpmdoofaelbinaeodfpjheijkbh) to live-reload my page as I work. As far as I'm aware, there's a bunch of similar tools and situations where something will make HEAD requests to our static files.

I've added a `HEAD` handler to the `RouterGroup.Static` function that replies with the `Content-Type` and `Last-Modified` of the request resource. With that browser extension and this function, I can see updates to my static files instantly in my browser!