code: public static <T> Transformer<T, T> delayFinalUnsubscribe(long duration, TimeUnit unit) {
          return delayFinalUnsubscribe(duration, unit, Schedulers.computation());
      }
docs: If multiple concurrently open subscriptions happen to a source
  transformed by this method then an additional do-nothing subscription
  will be maintained to the source and will only be closed after the
  specified duration has passed from the final unsubscription of the open
  subscriptions. If another subscription happens during this wait period
  then the scheduled unsubscription will be cancelled.
  
  @param duration
  duration of period to leave at least one source subscription
  open
  @param unit
  units for duration
  @param <T>
  generic type of stream
  @return transformer