Roll back Grack::Auth to fix Git HTTP SPNEGO

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/728

This MR keeps the new Git HTTP request specs, which still pass. It routes Git HTTP traffic back to Grack::Auth so that we get Kerberos SPNEGO support back. Just to be sure we also restore the (isolated) Grack::Auth spec file that contains the only tests for Kerberos SPNEGO we currently have.