Islandora/islandora — pull request #473 by adam-vessey (2014-04-04 13:46:06+00:00)
Title: Make the "undo_function" bit optional.

The "undo_function" part of callbacks was required, even though it
might not be required for functionality... Make it optional, so we
don't end up making/using no-op functions to avoid doing anything.