options usually doesn't have a `hash` specified. This will usually get calculated on line [166](https://github.com/mozilla/PyHawk/blob/4c7dc682eeba88b3d065e477b37b466f81b0bfbb/hawk/server.py#L166).

Sorry I don't have automated unit tests yet, but if you start up `sample_server.py` and `sample_client.py`, you will see that this pull request will raise a `KeyError`.

The reason I put the check in here is because it is what I've interpreted as [hawk's behavior](https://github.com/hueniverse/hawk/blob/a523a412afbda2eca1fb1d253b8e3511fdbd371d/lib/server.js#L270).

Thanks for the pull request, the review is appreciated.