Pass settings through `maintain_nested_settings` callback.

Without this, new default settings would not be added to the `shortcode_atts()` output, resulting in an 'undefined index' notice on the Settings page.


git-svn-id: https://plugins.svn.wordpress.org/tagregator/trunk@1006862 b8457f37-d9ea-0310-8a92-e5e31aec5664