title: Knockout JS valueUpdate keypress not working for enable binding
body: <p>I have a 'Save' button on my form that has knockout enable binding. I added valueUpdate: 'keypress' to my knockout binding expression hoping for save button to become enabled on keypress.  I have a simple input textbox with value knockout binding.  Instead, page still requiring user to click away for save button to become enabled. HTML below. when user types in input, canSave becomes true enabling save button.</p>
  
  <pre><code>&lt;button data-bind="click: save, enable: canSave, valueUpdate: 'keypress'"&gt;Save&lt;/button&gt;
  
  &lt;h3&gt;&lt;strong&gt;First Name: &lt;input data-bind="value: firstName"&gt;&lt;/strong&gt;&lt;/h3&gt;
  </code></pre>