Don't clone the jsdom config object unnecessarily (#13)

Fixes #11

src/index.js

@@ -4,10 +4,8 @@ const { JSDOM } = require('jsdom');
 
 // Class to return a window instance.
 // Accepts a jsdom config object.
-// Config object must be cloned before passing through otherwise jsdom will add
-// lots of properties to the original reference.
 module.exports = class Window {
 	constructor(jsdomConfig) {
-		return (new JSDOM('', Object.assign({}, jsdomConfig))).window;
+		return (new JSDOM('', jsdomConfig)).window;
 	}
 };