Bug 16573: change created_on type - atomic update

Bug 16573: change created_on type - atomic update

This patch change 'created_on' virtualshelves column to
datetime type

To test:
1) Apply the patch
2) Run updatedatabase.pl
3) Verify type of 'created_on' column

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 162b9015d9bfb23200d1bddff69be9384a33463b)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>