rxgroups/src/main/java/com/airbnb/rxgroups/ObservableGroup.java
public <T> void resubscribe(TaggedObserver<? super T> observer) {
    resubscribe(observer, Utils.getObserverTag(observer));
  }
public <T> void resubscribe(TaggedObserver<? super T> observer) {
    resubscribe(observer, Utils.getObserverTag(observer));
  }
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}.