Conforming to Django 1.6+ special timeout values

Django 1.6+: Explicitly passing in timeout=None will set a non-expiring timeout. Passing in timeout=0 will set-and-expire-immediately the value.

I didn’t know tox. The tox output was so verbose that I can’t tell if my commit passed the tests or not…
It would appear that the following `self.set(..)` call will use `-1` timeout. This means that if you `.add(key, val, None)` (no expiry) the inner set call will be `.set(key, value, -1)` (immediate expiry). Is this a bug? Can you explain?