code: public <T> void resubscribe(TaggedObserver<? super T> observer) {
      resubscribe(observer, Utils.getObserverTag(observer));
    }
docs: Resubscribes the {@link TaggedObserver} to the observable
  identified by {@code observableTag}.
  <p> Convenience method
  for {@link #resubscribe(TaggedObserver, String)} when {@code observer} only
  is subscribed to one {@link Observable}.