Fix database_field_name for nil
Update changelog
Fix namespacing issue in mode generator.

[ fix #2886 ]
Enable public-facing dirty attribute methods to cope with field aliases

Changeable uses db field names internally but should also be able to handle
aliases given to its public-facing methods.
(cherry picked from commit 9b67b050b3c7265d4d055a0d68029a6f4059ebc9)