util: use Object.create(null) for dictionary object

Fixes https://github.com/nodejs/node/issues/3788
`arrayToHash()` needs to use `Object.create(null)` for its dictionary object.

Refs: https://github.com/nodejs/node/pull/3791
PR-URL: https://github.com/nodejs/node/pull/3831


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>