title: CakePHP: Setting default value on a select with optgroups
body: <p>On a normal select, with no optgroups, the following code in CakePHP would make a value the default one:</p>
  
  <blockquote>
    <p>'selected' => $value</p>
  </blockquote>
  
  <p>Once I have optgroups (a select tag with headings) how do I set a default value? The previous code doesn't seem to work.</p>