title: Knockoutjs does not update value when select options changed
body: <p>Problem is when select options got changed value is not being updated.</p>
  
  <p>So when I change Honda to Toyota, options in second select got changed fine, but selectedModel observable not. I must have missed something. </p>
  
  <p>JavaScript:</p>
  
  <pre><code>function ViewModel() {
      var self = this;
  
      self.selectedMake = ko.observable()
      self.selectedModel = ko.observable()
  
      self.makes = ["Honda", "Toyota"];
      self.models = ko.computed(function () {
          if (self.selectedMake() === "Honda") return ["CRV", "Accord"];
          if (self.selectedMake() === "Toyota") return ["Rav4", "Camry"];
          return [];
      });
  }
  $(function () {
      ko.applyBindings(new ViewModel());
  });
  </code></pre>
  
  <p>HTML:</p>
  
  <pre><code>&lt;select data-bind="value: selectedMake, options: makes"&gt;&lt;/select&gt;
  &lt;select data-bind="value: selectedModel, options: models"&gt;&lt;/select&gt;
  &lt;p&gt;Selected make: &lt;b data-bind="text:selectedMake"&gt;&lt;/b&gt;&lt;/p&gt;
  &lt;p&gt;Selected model: &lt;b data-bind="text:selectedModel"&gt;&lt;/b&gt;&lt;/p&gt;
  </code></pre>
  
  <p>JS Fiddle: <a href="http://jsfiddle.net/apuchkov/n4VyD/" rel="noreferrer">http://jsfiddle.net/apuchkov/n4VyD/</a></p>