gerrit: Add support for Digest Authorization

The existing implementation doesn't support the HTTP Digest Authorization
that widely used in docker-based Gerrit configuration

Proposed code is based on http://play.golang.org/p/ABoHSHoTmu

Change-Id: Ia01d03cc849a4fcd538b05a60b83ac7e18809d5a
Reviewed-on: https://go-review.googlesource.com/29295
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed