title: Having my SELECT with optgroup + optionsCaption with knockout seems to be ignored
body: <p>I have a solution for using an optgroup with my SELECT html tag.</p>
  
  <p>Here is the SO post: <a href="https://stackoverflow.com/questions/15948619/construct-a-knockout-select-with-optgroup-based-on-observablearray">Construct a knockout SELECT with optgroup based on observableArray</a></p>
  
  <p><img src="https://i.stack.imgur.com/OOckD.png" alt="enter image description here"></p>
  
  <p>Now I would like to allow an empty value in this SELECT. I try with optionsCaption:</p>
  
  <pre><code>&lt;select data-bind="foreach: brands, optionsCaption: '  '"&gt;
      &lt;optgroup data-bind="attr: {label: $data}, foreach: $parent.vehicles"&gt;
      &lt;!-- ko if: Brand == $parent --&gt;
          &lt;option data-bind="text: Type"&gt;&lt;/option&gt;
      &lt;!-- /ko --&gt;
      &lt;/optgroup&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>But it doesn't work: no empty option.</p>
  
  <p>Any idea?</p>
  
  <p>Thanks.</p>