title: CombineLatest and cold observables
body: <p>Given the following code:</p>
  
  <pre class="lang-java prettyprint-override"><code>        Observable&lt;String&gt; obs1 = Observable.just("1", "2", "3");
          Observable&lt;String&gt; obs2 = Observable.just("a", "b");
          Observable.combineLatest(obs1,
                                   obs2,
                                   (s1, s2) -&gt; s1 + ":" + s2
          ).subscribe(System.out::println);
  </code></pre>
  
  <p>I would expect it to print something like the following:</p>
  
  <pre><code>1:a
  2:a
  3:a
  3:b
  </code></pre>
  
  <p>But it prints</p>
  
  <pre><code>3:a
  3:b
  </code></pre>
  
  <p>Why is my first Observable only emitting the last item? There are no active subscriptions before combineLatest, so it should be cold. How can I make sure all items from both Observables are combined together?</p>