Fix deepEqual for ES6 Object.keys

`Object.keys` is changed in ES6. It no longer throws errors for primitives.
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.keys

Next Chrome 40 and Firefox 35 will change `Object.keys` for ES6.

* Chrome (V8): https://code.google.com/p/v8/issues/detail?id=3443
* Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1038545

and then, `assert.deepEqual` will be broken.

FYI: io.js has fixed same issue https://github.com/iojs/io.js/pull/193