Support the X-Forwarded-Proto header for proxies

For example, a nginx reverse proxy that handles the TSL itself. Therefore, the
requests to the express server is done over HTTP but the well-known document
should reflect the https of the original request.
Adds test for X-Forwarded-Proto support
Merge pull request #1 from OADA/forwarded-proto

Support the X-Forwarded-Proto header for proxies