2.x Calling hasNext() on a disposed blocking iterator hangs forever

Calling `hasNext()` on a a blocking iterator never returns if the iterator has been disposed. 

Tested on 2.2.11

To reproduce:
```
Iterator<Integer> iterator = Flowable.fromArray(1, 2, 3).delay(1, TimeUnit.SECONDS).blockingIterable().iterator();

System.out.println("start");
((Disposable)iterator).dispose();
iterator.hasNext(); //Hangs forever
```
