title: preselect multiple values using the selectedOptions knockout binding in an select with optgroup
body: <p>Ello, </p>
  
  <p>I've litteraly tried all the options to making a (multiple) selectbox with optgropus, and binding the options/selectedOptions with knockout. </p>
  
  <p>There seems to be an issue with the selectedOptions binding. My data seems to be legit, but it just won't show the preselected options.</p>
  
  <p>I've made an example in JSFiddle: <a href="http://jsfiddle.net/aCS7D/251/">http://jsfiddle.net/aCS7D/251/</a></p>
  
  <pre><code>&lt;select data-bind="selectedOptions: selectedOptions" multiple="multiple"&gt;
  &lt;option&gt;Please choose an option&lt;/option&gt;
  &lt;optgroup data-bind="repeat: groups" data-repeat-bind="attr: {label: $item().label}"&gt;
      &lt;option data-bind="repeat: $item().children" data-repeat-bind="text: $item().label, option: $item()"&gt;&lt;/option&gt;
  &lt;/optgroup&gt;
  </code></pre>
  
  <p></p>
  
  <p>With a single selected option it works, but with multiple selectedoptions, the selectbox does not render them correctly.</p>
  
  <p>If any one has a solution to this problem you would be my hero! </p>