Revert "There is already a Set of non-hidden action_names lying around."

Fixes a regression where action_missing was no longer working. The
change introduced the assumption that all controller actions are known
beforehand, and there were no tests stating otherwise.

This reverts commit 7cc5bf51b4c16e89340b8c45ca0297e387226aaf.
Add action_missing test coverage; update CHANGELOG