Whenever we have compat fields to render, send a dummy menu_order field (which was always sent in 3.4) to ensure an unchecked checkbox can still be processed by attachment_fields_to_save.

Merges [23290] to the 3.5 branch.
fixes #22868.



git-svn-id: http://core.svn.wordpress.org/branches/3.5@23291 1a063a9b-81f0-0310-95a4-ce76da25c4cd