net/http/httputil: add X-Forwarded-{Host,Proto} headers in ReverseProxy

net/http/httputil: add X-Forwarded-{Host,Proto} headers in ReverseProxy

X-Forwarded-Host contains the original request's host.

X-Forwarded-Proto contains "http" or "https", depending on whether the
original request was made on a TLS-secured connection.

Setting either header to nil in Director disables adding the header,
same as for X-Forwarded-For.

Fixes #50465.

Change-Id: If8ed1f48d83f8ea0389c53519bc7994cb53891db
Reviewed-on: https://go-review.googlesource.com/c/go/+/407414
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>