Check X-Forwarded-Proto when behind a proxy to see what the protocol is

Hey, here is a tiny change to support check X-Forwarded-Proto when behind a proxy

Both https://metacpan.org/pod/Dancer::Deployment#Using-Nginx and http://en.wikipedia.org/wiki/List_of_HTTP_header_fields#cite_ref-17 suggest this is the correct header to check but the code wasn't checking it