PIM-3244 : fix memory leak on products with media export
PIM-3244 : re-work memory leak fix to ensure backward compatibility
Merge pull request #1640 from akeneo/fix/PIM-3244

Fixed PIM-3244 memory leak on products with media export
PIM-3228 : prepare tag 1.2.8
PIM-3228 : update version to 1.2.x
PIM-3221: Fix bug on variant group import

*Fix bug on variant group import, possibility to update variant
groups's attributes on import and it shouldn't be possible