assert: fix deepEqual regression

Change of Object.keys in ES6 breaks assert.deepEqual about primitive
values.

V8: https://code.google.com/p/v8/issues/detail?id=3443

Previously deepEqual depends on Object.key that throws an error for
a primitive value, but now Object.key does not throw.

PR-URL: https://github.com/iojs/io.js/pull/193


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>