CASC-232: x-forwarded-host not properly handled when generating service url

Fix for https://issues.jasig.org/browse/CASC-232

Fixes building of service url when X-Forwarded-* headers are present indicating that the request was proxied.  In this case, the service url should be generated for the original request, not the proxied request.
Interesting option.   Assuming we have sufficient code coverage, could be a good thing to try.