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.
Ok, merged the tests. (By the way, doing the equals check both directions made me find an actual bug, wohoo!) And yes, I missed the type. Thanks.