Repeat/Concat/Zip Backpressure

This code throws `MissingBackpressureException`:

``` java
        Observable<Integer> o1 = Observable.from(1, 1).concatWith(Observable.just(-1).repeat());
        Observable<Integer> o2 = Observable.from(2, 2, 2, 2);
        Observable.zip(o1, o2, (a, b) -> a + b).forEach(System.out::println);
```

It should emit `3, 3, 1, 1` and end correctly.
