Ensure there is a unit test for every property.

[pak21]

Too long a day at work to think about the big PR, so here's a simple one: ensure that there is a unit test for every property which exists on the BoardGame object. This is particularly important if we're going to refactor any of this in the near future :-)

Interestingly, this reveals what might actually be a bug: the BoardGame.users_wanting, BoardGame.users_wishing and BoardGame.users_commented properties are trying to fetch data from self._data rather than self._stats and always seem to be None. If you agree this is a bug, I'll do a separate PR for that.

[lcosmin]

Yes, it's a bug. It has gone unnoticed because at the beginning the tests were done using the real BGG API and I just ignored values which could change from call to call (probabilities, averages, etc). After switching to mocked responses from the API, I simply forgot to add tests for everything.

I guess that, since nobody reported this issue, nobody uses those properties 😄

Yeah, sure, send a PR, I'll merge it 👍