Block bad keys in _mergerIf

src/helpers/helpers.core.js

@@ -1,5 +1,9 @@
 'use strict';
 
+function isValidKey(key) {
+	return ['__proto__', 'prototype', 'constructor'].indexOf(key) === -1;
+}
+
 /**
  * @namespace Chart.helpers
  */
@@ -196,7 +200,7 @@ var helpers = {
 	 * @private
 	 */
 	_merger: function(key, target, source, options) {
-		if (['__proto__', 'prototype', 'constructor'].indexOf(key) !== -1) {
+		if (!isValidKey(key)) {
 			// We want to ensure we do not copy prototypes over
 			// as this can pollute global namespaces
 			return;
@@ -217,6 +221,12 @@ var helpers = {
 	 * @private
 	 */
 	_mergerIf: function(key, target, source) {
+		if (!isValidKey(key)) {
+			// We want to ensure we do not copy prototypes over
+			// as this can pollute global namespaces
+			return;
+		}
+
 		var tval = target[key];
 		var sval = source[key];