add backpressure support for defaultIfEmpty()

[davidmoten]

When looking at implementing backpressure support for defaultIfEmpty the obvious strategy is to reuse switchIfEmpty and get rid of OperatorDefaultIfEmpty. That's what this PR does.

There is a little bit of overhead with using Observable.from(Arrays.asList(defaultValue)) as the alternate observable. Might be nice to have a version of Observable.just(item) that supports backpressure to use in its place (you've mentioned this before @akarnokd).

[akarnokd]

You could just create an Observable inline:

public final Observable<T> defaultIfEmpty(T defaultValue) {
    //if empty switch to an observable that emits defaultValue and supports backpressure
    return switchIfEmpty(Observable.create(s -> {
        s.setProducer(new SingleProducer<>(defaultValue));
    }));
}

[davidmoten]

Great, can do. I'd like to see that as its own method. I'll put that change in and open another issue so method names/overrides and desirability can be discussed.

[akarnokd]

Thanks!