Fix HTTP digest authentication

Fix HTTP digest authentication

JGit's internal implementation of the HTTP digest authentication
method wasn't conforming to RFC 2617 (HTTP Authentication: Basic
and Digest Access Authentication), resulting in authentication
failures when connecting to a digest protected site.

The code now more accurately matches section 3.2.2 (The Authorization
Request Header) from the standards document.

Change-Id: If41b5c2cbdd59ddd6b2dea143f325e42cd58c395
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>