forum_submit reverts changes to non-forum taxonomy

If forum nodes can be tag with terms in addition to those in the Forum vocabulary, then a bug manifest as follows:

Edit an existing forum topic. Change the the non-forum taxnomy categories and choose "leave a shadow copy". regardeless of whether the forum topic is actually being moved, the changes to the non-forum taxonomy will be discarded due to faulty logic in forum_submit. Note that all the taxonomy terms are replaced by what's loaded from {term_node} via the call to taxonomy_node_get_terms.

function forum_submit(&$node) {
  // Make sure all fields are set properly:
  $node->icon = $node->icon ? $node->icon : '';

  if ($node->taxonomy) {
    // Extract the node's proper topic ID.
    $vocabulary = variable_get('forum_nav_vocabulary', '');
    foreach ($node->taxonomy as $term) {
      if (db_result(db_query('SELECT COUNT(*) FROM {term_data} WHERE tid = %d AND vid = %d', $term, $vocabulary))) {
        $node->tid = $term;
      }
    }
    if ($node->tid && $node->shadow) {
      // A shadow copy needs to be created. Retain existing term and add new term.
      $terms = array_keys(taxonomy_node_get_terms($node->nid));
      if (!in_array($node->tid, $terms)) {
        $terms[] = $node->tid;
      }
      $node->taxonomy = $terms;
    }
  }
}