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.
yeah. It's the code that is wrong though, not the comment. (well, the comment is also wrong in that it says "item" rather than "property")

You could have seen this by checking the `Comparable` interface or the implementation in `Item`.