autosave plugin stores content only if it was really changed #6629

This fixes problematic behavior when editing text as described in bug #6629 (http://www.tinymce.com/develop/bugtracker_view.php?id=6629)

Content was always auto-saved, which means if the editor is loaded with stored text from the server, eventually it would be auto-saved over any previous modifications that might exist, rendering the recovery button useless.

The solution is to auto-save only if the content was modified (editor is dirty).