use minifiedjs extend() instead of Object.assign()

test/fixture.js

@@ -1,5 +1,6 @@
 'use strict';
 
+var _ = require('../src/scripts/vendor/minified/minified')._;
 var idCounter = 0;
 
 function fixture(type, config) {
@@ -13,7 +14,7 @@ function fixture(type, config) {
 
   idCounter++;
 
-  return Object.assign({}, basic, config);
+  return _.extend({}, basic, config);
 }
 
 module.exports = fixture;