Unregister view when replaced in Presenter

Consider the following case:
1. View A in scope A calls mySingletonPresenter.takeView()
2. View B in scope B (say, a fragment or activity with a different scope) calls mySingletonPresenter.takeView()
3. scope A is destroyed

In the case above, the onDestroy() method in Presenter will nullify the view, even though View B has replaced View A.  The problem is that we register a view in takeView(), but we do not unregister the view in dropView().

Since MortarScope#unregister() does not, you know, exist, we do the next best thing.  We check that the registration has not been replaced before calling into the Presenter.