Allow authGSSClientInit principal kwarg to be None.

[kwlzn]

This PR simply applies the patch from #49 for easier merging.

Fixes #49

[dreness]

Hi @kwlzn

We don't yet have any automated tests hooked up for this project. Some of the included tests require admin-level access to a kerberized service of some variety. If you happen to have such a configuration at your disposal, would you mind running through the included tests (with your patch applied) and posting your results?

I just added some testing notes to clarify how the tests are used, and the requirements / preconditions for each. Normally I wouldn't ask you to do this, but since I don't have an environment handy for this, and since you asked for expediting, this might be a chance to speed things along a bit. If not, that's fine too, I will just need to set up a test environment (which shouldn't take all that long...).

[kwlzn]

@dreness I unfortunately do not have admin-level access within our infra, sorry. :(

[dreness]

Just spent a lot of hours trying to get the tests to run against a system which I am now declaring to be too complicated for this purpose (apple's Server.app web service). I will try to find the simplest docker thing that I can find instead; unfortunately I don't remember exactly what I used last time.

[cyrusdaboo]

This change looks OK to me.

[cyrusdaboo]

Looks OK.