Why is `ResourceSubscriber` using `CompositeDisposable` and `ResourceObserver` using `ListCompositeDisposable`? Also the java documentation does not seem to fit of the disposables. `/** The resource composite, can be null. */` It's initialized and made final and adding a `null` Disposable will throw a `NullPointerException`. In addition is there a need for ResourceMaybeObserver and the alike implementation for Single & Completable?