OperatorObserveOn should not request more after child is unsubscribed

[davidmoten]

In existing code if unsubscription happens immediately after a child.onNext call from pollQueue then the number of emissions made in the loop will be requested of upstream despite child being unsubscribed.

This PR adds a local variable isUnsubscribed and when set to true no further requests will be made.

This PR includes a unit test that failed on the existing codebase.

[akarnokd]

Less verbose:

for (;;) {
    if (child.isUnsubscribed()) {
        return;
    }
    // ...

[davidmoten]

Yeah, that's better. Done

[akarnokd]

Thanks!