There was regression introduced by #28580 where only changed (dirty) settings now are POST'ed to the Customizer preview.
* Allow WP_Customize_Manager::post_value() to accept a second $default argument.
* Introduce WP_Customize_Manager::unsanitized_post_values() for accessing previously-private member variable _post_values.
* Do require_once instead of require for Customizer classes.
* Add unit tests for WP_Customize_Manager and WP_Customize_Setting.
props westonruter.
fixes#30988.
git-svn-id: https://develop.svn.wordpress.org/trunk@31329 602fd350-edb4-49c9-b593-d223f7449a82