title: rxjava emit inner observable once it starts emitting
body: <p>Is there any easy way to create an observable of observables that will only emit once the inner observable emits? For example:</p>
  
  <p>say we have a magical observable that does this</p>
  
  <pre><code>Observable&lt;Observable&lt;Integer&gt;&gt; magicOuterObservable;
  </code></pre>
  
  <p>subscribe to outer</p>
  
  <pre><code>magicOuterObservable.subscribe(subscriber)
  </code></pre>
  
  <p>offer inner observable to outer</p>
  
  <pre><code>magicOuterObservable.onNext(inner);
  </code></pre>
  
  <p>nothing yet</p>
  
  <pre><code>subscriber.assertNoValues();
  </code></pre>
  
  <p>offer value to inner observable</p>
  
  <pre><code>inner.onNext(1)
  </code></pre>
  
  <p>outer emits</p>
  
  <pre><code>subscriber.assertValue(inner);
  </code></pre>
  
  <p>Terminal events would not cause an emission. Only the first inner emissions really.</p>
  
  <p>Any ideas?</p>
  
  <p>If anyone is curious, this for a potentially over-engineered pull to refresh handler.</p>