Add support for X-Forwarded-Proto headers

I have added support for `X-Forwarded-Proto`

When using Amazon AWS, and off-loading SSL at the AWS Load Balancer, the X-Forward headers are [limited] (http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/TerminologyandKeyConcepts.html#x-forwarded-headers) to `X-Forwarded-Proto`, as these machines are not configurable by users.