Add Object.freeze on private enums

Improve code readability

src/systemConsumerStrategies.js

@@ -1,8 +1,7 @@
-const SystemConsumerStrategies = {
+const SystemConsumerStrategies = Object.freeze({
   DispatchToSingle: 'DispatchToSingle',
   RoundRobin: 'RoundRobin',
   Pinned: 'Pinned'
-};
-Object.freeze(SystemConsumerStrategies);
+});
 
-module.exports = SystemConsumerStrategies;
+module.exports = SystemConsumerStrategies;