title: RxJava PublishSubject+Debounce: the second item not emited
body: <p>I want to use PublishSubject + debounce (in subscribe logic) for emit my items with delay. This is my code:</p>
  
  <p>Subscription logic:</p>
  
  <pre><code>notificationSubject = PublishSubject.create&lt;Notification&gt;()
  notificationSubject
              .debounce(300, TimeUnit.MILLISECONDS)
              .doOnIOSubscribeOnMain() // ext. fun, I hope you understand it
              .subscribe {
                  displayNotification(it)
              }
  </code></pre>
  
  <p>And emit objects logic:</p>
  
  <pre><code>showNotification(obj1)
  showNotification(obj2) 
  // ...
  fun showNotification(notification: Notification) {
      notificationSubject.onNext(notification)
  }
  </code></pre>
  
  <p>But on subscribe I receive only first emitted item (obj1). And if I emit two objects (obj3, obj4) again I receive only first of emitted item (obj3).</p>
  
  <p><strong>How to fix it?</strong> </p>