code: @SuppressWarnings("unchecked")
      @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Flowable<T> startWith(T value) {
          ObjectHelper.requireNonNull(value, "item is null");
          return concatArray(just(value), this);
      }
docs: Returns a Flowable that emits a specified item before it begins to emit items emitted by the source
  Publisher.
  <p>
  <img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/startWith.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator honors backpressure from downstream. The source {@code Publisher}
  is expected to honor backpressure as well. If it violates this rule, it <em>may</em> throw an
  {@code IllegalStateException} when the source {@code Publisher} completes.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code startWith} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param value
  the item to emit first
  @return a Flowable that emits the specified item before it begins to emit items emitted by the source
  Publisher
  @see <a href="http://reactivex.io/documentation/operators/startwith.html">ReactiveX operators documentation: StartWith</a>