title: RxJava 2 - Observable.zip() never emits
body: <p>I'm new with Reactive.
  I want to collect data from different sources and process the result.
  To do that I'm using the <em>Observable.zip()</em> method. But it never emits so the callback is never called.
  What have I done wrong?</p>
  
  <p>Here is an example of the code I try to implement:</p>
  
  <pre><code>public void loadData() {
  
      Observable.zip(
              Observable.just(42),
              Observable.just(43),
              Observable.just(44),
              (integer, integer2, integer3) -&gt;
                      Log.d(TAG, "zip method called") // This is never called
      );
  }
  </code></pre>