code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Completable mergeWith(CompletableSource other) {
          ObjectHelper.requireNonNull(other, "other is null");
          return mergeArray(this, other);
      }
docs: Returns a Completable which subscribes to this and the other Completable and completes
  when both of them complete or one emits an error.
  <p>
  <img width="640" height="442" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.mergeWith.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code mergeWith} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param other the other Completable instance
  @return the new Completable instance
  @throws NullPointerException if other is null