cmd/go/internal/web: log complete URLs

Incoming URLs may omit the scheme to indicate “either HTTP or HTTPS”.
For such URLs, log the scheme actually used instead of leaving it out.

(This issue was noticed while triaging #34075.)

Updates #34075

Change-Id: I39e5ca83543dd780258d41d5c2c4ba907cd20e5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193262


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>