public static <I, P, R> WorkerTask<I, P, R> of(@NonNull final String globalId, @NonNull final Function<I, Observable<TaskValue<P, R>>> taskSupplier) {
        return of(globalId, taskSupplier, null, null);
    }
registers an observer not bound to a lifecycle. This method is used for Unit-Tests
main/src/main/java/cgeo/geocaching/utils/workertask/WorkerTask.java