util: faster arrayToHash

The `util.format()` is used frequently, make the method faster
is better.

R-URL: https://github.com/nodejs/node/pull/3964


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>