toSingle() leaks subscriber if followed by flatmap.

[RoryKelly]

Not sure this is a correct test, let me know if I can provide any extra information.

    private Subscriber<? super Object> subscriber;

    @Test public void thisTestPasses() throws Exception {

        final Subscription subscribe = Observable.create(subscriber -> {
            this.subscriber = subscriber;
        }).subscribeOn(Schedulers.io()).flatMap(o -> Observable.just("")).toSingle().subscribe();

        subscribe.unsubscribe();

        assertThat(subscriber.isUnsubscribed()).isTrue();
    }

    @Test public void thisTestFails() throws Exception {

        final Subscription subscribe = Observable.create(subscriber -> {
            this.subscriber = subscriber;
        }).subscribeOn(Schedulers.io()).toSingle().flatMap(o -> Single.just("")).subscribe();

        subscribe.unsubscribe();

        assertThat(subscriber.isUnsubscribed()).isTrue();
    }

[akarnokd]

Thanks for reporting. Indeed, this is a bug in Single.flatMap. Fix PR in #3941 .