title: SelectList with SelectListGroup
body: <p>I'm trying to return a <code>SelectList</code> with <code>&lt;optgroup&gt;</code>'s using <code>SelectListGroup</code>, but the code below only returns the values in the SelectList without the groups. How can I have my select list separated by groups?</p>
  
  <pre><code>public SelectList MyList()
  {
     var group1 = new SelectListGroup() { Name = "Group 1" };
     var group2 = new SelectListGroup() { Name = "Group 2" };
  
     var items = new List&lt;SelectListItem&gt;();
  
     items.Add(new SelectListItem() { Value = "1", Text = "Item 1", Group = group1 });
     items.Add(new SelectListItem() { Value = "2", Text = "Item 2", Group = group2 });
  
     return new SelectList(items, "Value", "Text");
  }
  </code></pre>