Daniel Hahler: Handle X-Forwarded-Proto in backend nginx

When nginx is being used as backend server, it will talk plain "http" to the frontend server only (for performance reasons and to simplify setup).

But web applications often need to know, if the traffic between the browser and server is encrypted (https), e.g. when checking if a particular part of the site is being accessed securely.

Since only the frontend proxy (the one between nginx and the browser) knows about this, this information has to be forwarded to the backend.