code: public static Completable toCompletable(Publisher<?> publisher) {
          if (publisher == null) {
              throw new NullPointerException("publisher");
          }
          return Completable.create(new PublisherAsCompletable(publisher));
      }
docs: Converst a Publisher into a Completable by ignoring all onNext values and emitting
  onError or onComplete only.
  @param publisher the Publisher instance to convert
  @return the Completable instance
  @since 1.1
  @throws NullPointerException if publisher is null