swagger: honour X-Forwarded-Proto header

Improve the algorithm building `baseUrl` to honour `X-Forwarded-Proto` header when it is present.

Close #57 

See also https://github.com/shelbys/strong-remoting/commit/c049f134214e0c42ce65c77483feeb15df79b9d9

/to @raymondfeng 
/cc @shelbys