HHH-7918 - Merging ModWorkUnits ended with incorrect values of modified flags

Patch for HHH-7918 JIRA ticket.
Link: https://hibernate.onjira.com/browse/HHH-7918

@mskowron, @adamw: Please review. I know that we could just alter ``data`` variable in the second ``ModWorkUnit`` while merging, but thought that creating new object would be a cleaner way.

Regards,
Lukasz Antoniak