Fixes for Count Side Effect Operators

This release contains bug fixes for the doOnNextCount(), doOnErrorCount(), and doOnCompleteCount() transformers as well as their FX counterparts doOnNextCountFx(), doOnErrorCountFx(), and doOnCompleteCountFx().

Unit tests have been added for these operators as well.

       List<Integer> onNextCounts = new ArrayList<>();

        Observable.just("Alpha", "Beta", "Gamma")
                .compose(FxObservableTransformers.doOnNextCount(onNextCounts::add))
                .subscribe();

        Assert.assertTrue(onNextCounts.containsAll(Arrays.asList(1, 2, 3)));