Fix #2558 Undecorate removes non-mobx decorators

[urugator]

Fix for #2558

[changeset-bot]

⚠️ No Changeset found

Latest commit: 89966dc

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[coveralls]

Coverage remained the same at 95.542% when pulling 89966dc on urugator:fix-2558 into 5402d09 on mobxjs:mobx6.

[danielkcz]

Same here, everything green, so let's push it forward, although release has to be handled manually (@mweststrate).