[unity] SkeletonDataAsset does not automatically update

[HaraldCsaszar]

As reported on the forum:
http://esotericsoftware.com/forum/SkeletonDataAsset-does-not-automatically-update-11816

Can be reproduced on Unity 2017.1 and 2018.3 with existing character with multiple separate atlases in scene when changing a slot color and re-exporting only the json file (not regenerating atlas files).
Steps:

1. export the goblins project once, with 3 separate atlas files in separate directories and the json file.
2. import in Unity, add character to scene as SkeletonAnimation
3. change head slot color in Setup mode in Spine, reexport only json file
4. override existing file in Unity by copying it over the existing file in the assets subdir
5. reimport will happen, changes the preview window, but does not (immediately) change the character in the scene.