title: Continue stream after subscribe
body: <p>I have two streams, that looks as follow:</p>
  
  <pre><code>const source1 = Rx.Observable.of([1,2,3,4,5])
    .map(() =&gt; "I am source 1")
    .do((x) =&gt; console.log(x));
  const sub1 = source1.subscribe((v) =&gt; console.log("Subscribe 2"));
  
  const source2 = Rx.Observable.fromEvent(document, 'click')
     .do(() =&gt; console.log("execute!"))
     .switchMapTo(source1)
     .do(() =&gt; console.log("After switch map."))
     .map((x) =&gt; "source2");
  
  source2.subscribe((x) =&gt; console.log(x));
  </code></pre>
  
  <p>Without click on document I've got following output:</p>
  
  <pre><code>"I am source 1"
  "Subscribe 2"
  </code></pre>
  
  <p>Further clicks the output is:</p>
  
  <pre><code>"execute!"
  "I am source 1"
  "After switch map."
  "source2"
  </code></pre>
  
  <p>How can I force the stream <code>source1</code> to subscribe by further clicks and after continue with stream <code>source2</code>?</p>
  
  <p>I am expecting after several clicks following output:</p>
  
  <pre><code>"I am source 1"
  "Subscribe 2"
  "execute!"
  "I am source 1"
  "After switch map."
  "source2"
  </code></pre>