Squashed commit of the following:

commit 4d544c37de85809ba24777775960b57e3dc97e6b
Author: Marc Bonsels <marc.bonsels@otrs.com>
Date:   Fri Jun 21 12:59:08 2013 +0200

    Fixed bug#9540 - AgentTicketOwner doesn't activate PreviousOwner if new owner is selected