Allow static asset serving from env variable (enhanced!)
Conflicts:
	railties/CHANGELOG.md

This makes rails behave properly when you serve static assets
and you have X-Sendfile headers enabled. Nevertheless in most
cases you should not rely on that and serve static assets with
a webserver like Apache or Nginx (as you already have it in
place anyway if you use X-Sendfile)

set a particular value for it. This allows servers to set it through
X-Sendfile-Type, read
https://github.com/rack/rack/blob/master/lib/rack/sendfile.rb for more
info. Anyways you can force this value in your production.rb