EAGLESIX-2651: SetUnion: add missing isAssociativeAndCommutative getter

lib/pipeline/expressions/SetUnionExpression.js

@@ -38,3 +38,7 @@ Expression.registerExpression("$setUnion", base.parse);
 proto.getOpName = function getOpName() {
 	return "$setUnion";
 };
+
+proto.isAssociativeAndCommutative = function isAssociativeAndCommutative() {
+	return true;
+};