util: fix invalid date output with util.inspect

Prevent util.inspect of throwing on date object with invalid date value.
It changed to output result of toString method call.

PR-URL: https://github.com/nodejs/node/pull/6504


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>