Fix incorrect authentication headers on cache digest requests

Fix incorrect authentication headers on cache digest requests

  login=NEGOTIATE can have an additional parameter specified,
   like login=NEGOTIATE:xxx

One test added in rev.12714 does not take this case into account and it
will send a garbage "login:password" (== "NEGOTIATE:xxx") to its peer
when requesting a digest.

This is a workaround patch to remove the broken Authentication headers
entirely. Support for Negotiate to the peer on these digest requests is
still needed.