Fix authentication type names broken by 0b5441a8

Fix authentication type names broken by 0b5441a8

0b5441a8 introduced an enum for authentication types and changed the
case of digest and basic authentication type names to all uppercase.
This broke digest authentication at least when using Gerrit as the git
server.

According to RFC2617 [1] "Basic" and "Digest" is the literal to be used
in authentication headers and not "BASIC" [1] and "DIGEST" [2].
According to RFC4559 "Negotiate" [3] is used for SPNEGO based
authentication.

[1] http://tools.ietf.org/html/rfc2617#page-5
[2] http://tools.ietf.org/html/rfc2617#page-8
[3] http://tools.ietf.org/html/rfc4559#page-3

Bug: 435866
Change-Id: I6173aff9352d7def225cafe2d73e5176ad40dff0
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>