Form at admin/structure/menu/item/[mlid]/edit does not save values

I was curious about the bug so I decided to investigate. A tricky nut to crack, but obvious once I saw the offending line. The problem is the following code in menu_edit_item in modules/menu/menu.admin.inc (line 271):

foreach (array('link_path', 'mlid', 'module', 'has_children', 'options') as  $key) {
  $form[$key] = array('#type' => 'value', '#value' => $item[$key]);
}

Solved by adding one line of code to the form alter:

unset($form['options']['#value']['attributes']);