title: Resetting dropdown by manual subscription doesn't work depending on what observable was subscribed to
body: <p>I searched online and tried to think hard and understand this behavior but I still can't figure out why this is the behavior. May be I am missing something basic here?</p>
  
  <p>I'd appreciate some pointers/resources/answers.</p>
  
  <p>Consider this fiddle - <a href="http://jsfiddle.net/rniemeyer/e3cTT/" rel="nofollow">http://jsfiddle.net/rniemeyer/e3cTT/</a></p>
  
  <p>Why does this reset the product selection</p>
  
  <pre><code>// Whenever the category changes, reset the product selection
  self.category.subscribe(function() {
      self.productName(undefined);
  });
  </code></pre>
  
  <p>While this does not?</p>
  
  <pre><code>// Whenever the category changes, reset the product selection
  self.categoryName.subscribe(function() {
      self.productName(undefined);
  });
  </code></pre>