Updated to set the page layout when rendering custom actions (same as ResourceController).  Ugly cut and paste, but moving determine_active_admin_layout to BaseController seemed uglier without bigger changes.
(cherry-picked from 9eb53e1)
Added new scenario to to ensure custom actions render the active admin layout.
(cherry-picked from 5eb853e)
Refactored #determine_active_admin_layout into the BaseController