title: Knockout custom binding for indeterminate checkbox won't work
body: <p>Let's put it short: This is my knockout custom binding for putting a checkbox in indeterminate state.</p>
  
  <pre><code>ko.bindingHandlers.nullableChecked = {
      update: function (element, valueAccessor) {
          var value = ko.utils.unwrapObservable(valueAccessor());
          if (value == null) element.indeterminate = true;
          ko.bindingHandlers.checked.update(element, function () { return value; });
      }
  };
  </code></pre>
  
  <p>If the initial value is <code>null</code> everything works fine and checkbox is put in indeterminate state but when I click the checkbox it doesn't seem to update the bound property's value to false/true accordingly. Am I missing something?</p>