Fix test for pre-es5 enviros

Fixes https://github.com/jashkenas/underscore/commit/319bdb101e8f656796903c38119e0600f8a7abbf#commitcomment-7261186
Also `_.keys` is a bad test here because it uses `hasOwnProp`, rendering the test invalid. This should be `deepEqual(_.extend({}, subObj), {c: 'd'})`