This is analogue to the `retrieve` method, which has the same signature. But I'm not so sure about this:

 * `retrieve` would be enough to only support `object_class` and `key`
 * Adding the `object` variant makes a bit easier to test
 * The `include?` would only need the former too, but asking if an object is in the map makes just sense. And using two different methods in that case seems to very Ruby.

Maybe we should just drop the `retrieve(object)` variant?