2.x: Add Disposable Observer for Maybe, Completable & Single

[vanniktech]

Also any reason the Dispoable is called s? Is it still from the Subscription time?

Fixes #4501

[vanniktech]

Seems unrelated:

io.reactivex.internal.operators.observable.ObservableRefCountTest > testRepeat FAILED
    java.lang.AssertionError: Error(s) present: [java.lang.IllegalStateException: onSubscribe not called in proper order] (latch = 1, values = 51, errors = 1, completions = 0)
        at io.reactivex.observers.TestObserver.fail(TestObserver.java:363)
        at io.reactivex.observers.TestObserver.assertNoErrors(TestObserver.java:411)
        at io.reactivex.internal.operators.observable.ObservableRefCountTest.testRepeat(ObservableRefCountTest.java:195)

        Caused by:
        io.reactivex.exceptions.CompositeException
            at io.reactivex.observers.TestObserver.fail(TestObserver.java:364)
            ... 2 more

            Caused by:
            io.reactivex.exceptions.CompositeException$CompositeExceptionCausalChain: Chain of Causes for CompositeException In Order Received =>

                Caused by:
                java.lang.IllegalStateException: onSubscribe not called in proper order

[akarnokd]

Yep, I'll fix that next - started to become annoying to me too.