title: Observable.Window and .Zip not functioning like I would expect
body: <p>I'm trying to turn an <code>IEnumerable</code> into an <code>IObservable</code> that delivers its items in chunks one second apart.</p>
  
  <pre><code>var spartans = Enumerable.Range(0, 300).ToObservable();
  
  spartans
      .Window(30)
      .Zip(Observable.Timer(DateTimeOffset.Now, TimeSpan.FromMilliseconds(1000)), (x, _) =&gt; x)
      .SelectMany(w =&gt; w)
      .Subscribe(
          n =&gt; Console.WriteLine("{0}", n),
          () =&gt; Console.WriteLine("all end"));
  </code></pre>
  
  <p>With this code, the only thing that is printed is "all end" after ten seconds. If I remove the <code>.Zip</code> then the entire sequence prints instantaneously, and if I remove the <code>.Window</code> and <code>.SelectMany</code> then the entire sequence prints one item per second. If I peek into the "windowed" observable inside the lambda passed to <code>SelectMany</code>, I can see that it is empty. My question is, why?</p>