Honor X-Forwarded-Proto In Redirects

When NS server is sat behind a load-balancer which is terminating SSL, a redirect will end up returning the wrong scheme, probably related to this bit of code here:

https://github.com/couchbase/ns_server/blob/15f8f4629bf26cd43f0009eb58256ef9e894e02f/src/menelaus_util.erl#L159

The load balancer should be operating in http mode, and inserting the X-Forwarded-Proto header which indicates the client is actually talking TLS.  In this case we need to return the https scheme if either the socket is TLS enabled, or the X-Forwarded-Proto header is set to https.

This may be required elsewhere too...