title: Disable a button if no save is needed
body: <p>With previous versions of Knockout, it was possible to deactivate a button if nothing needed to be saved like this:</p>
  
  <pre><code>&lt;button type="submit" data-bind="enable: CanSave"&gt;Save&lt;/button&gt;
  </code></pre>
  
  <p>With the 2.2.0 version, this result in an undefined error on this 'CanSave'.</p>
  
  <p>Any replacement?</p>
  
  <p>Thanks.</p>