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?
> You are right, but the test below does a little bit more than the data provider could do.

This is not really correct. We can change the `testNotEquals` method to check in both directions which I think is a good idea.

Furthermore, what about different data types?