Use utils.extend instead of Object.assign

... because javascript is awful

lib/utils.js

@@ -337,6 +337,9 @@ var deepCompare = module.exports.deepCompare = function(x, y) {
  *
  * All enumerable properties, included inherited ones, are copied.
  *
+ * This is approximately equivalent to ES6's Object.assign, except
+ * that the latter doesn't copy inherited properties.
+ *
  * @param {Object} target  The object that will receive new properties
  * @param {...Object} source  Objects from which to copy properties
  *