net/http: document setting of Proxy-Authorization header

net/http: document setting of Proxy-Authorization header

Add a test for setting a proxy username/password in
the HTTP_PROXY environment variable as well.

Fixes #61505

Change-Id: I31c3fa94c7bc463133321e9af9289fd47da75b46
Reviewed-on: https://go-review.googlesource.com/c/go/+/512555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>