Handle multiple X-Forwarded-Proto headers

Apparently getting proxied through setups like Haproxy -> nginx -> unicorn can set `HTTP_X_FORWARDED_PROTO` to a command delimited set of values like `https,https`. This splits on commas and ensures that the first protocol was `https`.

This is similar behavior to [rack-ssl](https://github.com/josh/rack-ssl/blob/0baafb18ca6080054f5041bb014d28e736cdceaf/lib/rack/ssl.rb#L42)'s behavior.

/cc @josh @bhuga @alindeman