Comment 15 for bug 1419823

Because in function do_add of ImageController, "path_root in image.extra_properties" is true and raise an ERROR.
And in function __setitem__ of ExtraPropertiesProxy, "self.__getitem__(key) is not None" is false and then do nothing.

BTW, I perfer to change this perperties both in add and replace op because none is as not set.