Closed Issue: document.selection is undefined when using ComboBox control [27552]

Basic CombBox implementation. Browser is IE 10. Using AjaxControlToolkit July 2013 edition -- also tried wthe September 2013 edition.

```
<form id="form1" runat="server">
<asp:ToolkitScriptManager ID="ScriptManager1" runat="server" CombineScripts="False" EnablePartialRendering="true"></asp:ToolkitScriptManager>
<ajaxToolkit:ComboBox ID="ComboBox1" runat="server">
<asp:ListItem Text="[Select an item]" Value="" />
<asp:ListItem Text="Actual Item #1" Value="SomeValue" />
<asp:ListItem Text="Actual Item #2" Value="3" />
<asp:ListItem Text="Actual Item #3" Value="xxx" />
</ajaxToolkit:ComboBox>
</form>
```

In Internet Explorer 10, when a key is pressed in the ComboBox, this exception is thrown.