issue 107 - Using spring-hateoas behind a HTTPS proxy.

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.
I've seen proxies return a `https,http` for the `X-Forwarded-Proto` header. Would it be more appropriate to change it to a case-insensitive `contains`?