Checking subscription before the execute method in UseCase.

Hi guys,

There is a leak in [UseCase](https://github.com/android10/Android-CleanArchitecture/blob/master/domain/src/main/java/com/fernandocejas/android10/sample/domain/interactor/UseCase.java) when you run `execute(Subscriber UseCaseSubscriber)` with different subscribers sequentially.

Maybe we need to check this subscription in this method and unsubscribe if we already have one?

What do you think?