Do not reauthenticate requests if connection is already authenticated

Currently,

every request on a single connection is reauthenticated, addtionally HTTPd-spawned subrequests produce duplicate headers.

Since GSS-API auth is connection-legged, it should be checked whether this connection has aready been authenticated. This branch does that. Tests with curl (single connection) and Firefox + IE (Windows) look fine.

This fixes issue #10.

In order to merge this commit, you need to process my other two PRs. If you like, I can rebase the relevant commit on top of your master.