Cannot add mixin to a locked node

Locking a node and trying to add a new mixin type while the node is locked results in a ConstraintViolationException.

There are a lock check in the addMixin(String mixinName) method which consider the lockowner and a lock check in the canAddMixin(String mixinName) method which only looks if the node is locked or not and returns false.

The lock check in the canAddMixin methods should also consider the lockowner.