The properties that are added with `.extend` should be enumerable

[3rd-Eden]

Currently when you extend a function and add new properties, these properties will not be enumerable by default.

function Foo() {}
Foo.extend = require('extendible');

var Bar = Foo.extend({
  hello: 'world'
});

var bar = new Bar();
console.log(Object.keys(bar)); // expecting `hello` to be in the array here.