lookup X-Forwarded-Proto for SSL

Actually, AWS ELB that uses SSL requests with "X-Forwarded-Proto" to behind app servers.
So, set URL base scheme to "https" if request header has "X-Forwarded-Proto: https".

About "X-Forwarded-Proto" on AWS ELB:
http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/TerminologyandKeyConcepts.html