Add more equals tests for Property

In #177 I said "more to come" which I never did. Property still misses some tests, at least an equals test. What about this?

__Edit:__ This also fixes an actual bug I found in `Property::equals`! It crashed for $this->id === null && $that->id !== null.
Heh - I thought I checked `Comparable` and saw the doc there. Looked again now and no doc. wtf?

> Didn't we talked about this weeks ago?

nope?