Merge pull request #1663 from akeneo/PIM-3266

PIM-3266: do not flush skipped items during an import
PIM-3282: fix the grid filters that can be set as json in the request
Merge pull request #1678 from akeneo/PIM-3282

PIM-3282: fix the grid filters that can be set as json in the request
Preparing version 1.2.9
Merge pull request #1680 from akeneo/version-1-2-9

Preparing version 1.2.9
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 (currently it display a
error message)

*Improved the behats