Menu attributes does not correctly use default values of text_format form elements

Problem/Motivation

Menu Attributes provides hook_menu_attributes_info to allow additional Form API elements to be included for saving within menu item options. This includes text_format Form API elements. However, text_format saves its value/format combination in the format

$text_format_value = array(
  'value' => $text_value,
  'format' => $text_format,
);

_menu_attributes_form_alter() does not account for this structure when setting the #default_value of its form elements, and so the saved text_format value/format combination is not loaded correctly into the form.

Proposed resolution

The attached patch has handling for the default value of text_format form elements.