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.
`Comparable` doesn't talk about this. I really wonder. It's not a real `equals` if it ignores a (very relevant) property. Didn't we talked about this weeks ago? Can't remember what the conclusion was.