Bug 22273: Database revision for table article_requests

Bug 22273: Database revision for table article_requests

Column created_on should not be updated. It is filled once by the
store method of Koha::ArticleRequest.
Column updated_on should be a timestamp, updated by the database.

Note: Although higher versions of MySQL and MariaDB support two timestamps
(NOT NULL) in one table, I kept on the safe side by allowing NULL on the
created_on column.

@RM: Don't forget to run DBIx update.

Test plan:
Run new install or upgrade.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>