Default value not working when select box uses optgroup grouping

When you create a select box using optgroups, e.g grouping options by a bold label within a select box, the default value always ends up in 'other' even though it's a valid list value.

e.g:

<?php
  // Make an optgroup style options array, grouped by vocabulary name
  foreach($result as $row){
    $options[$row->vocabulary_name][$row->tid] = $row->term_name;
  }
?>

Just to illustrate, the #options array ends up looking something like this:

Array
(
    [] => None
    [Opinion] => Array
        (
            [19] => Blogs
            [18] => Columns
            [20] => Debates
            [17] => Insight
            [22] => Letters
            [21] => Polls
        )

    [Lifestyle] => Array
        (
            [23] => Arts
            [24] => Beauty
            [25] => Books
            [26] => Education
            [27] => Entertainment
            [29] => Family
            [28] => Fashion
            [31] => Film
            [30] => Food
            [32] => Health
            [33] => Interiors
            [35] => Motoring
            [34] => Music
            [36] => Technology
            [37] => Travel
        )
    ...

I suspect it's to do with select_or_other_element_process
and the check

    if ($val
      && isset($element['select']['#options'])
      && is_array($element['select']['#options'])
      && !array_key_exists($val, $element['select']['#options'])
      && !in_array($val, $element['select']['#options'])) {

which may need to recurse into the #options array to find the value within the optgroup.

Curious if easy fix, else will need to workaround, possibly by overriding the _process function which would rather not do if patch available.

Lastly, thanks for the useful module!

DT