util: inspect boxed symbols like other primitives

Inspect boxed symbol objects in the same way other boxed primitives
are inspected.

Fixes: https://github.com/nodejs/node/issues/7639
PR-URL: https://github.com/nodejs/node/pull/7641


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>