module: remove unnecessary JSON.stringify

`debuglog` uses `%j` as a placeholder for replacement with
`JSON.stringify`. So that `JSON.stringify` is only called when the
appropriate debug flag is on. The other `%s` changes are for style
consistency.

PR-URL: https://github.com/nodejs/node/pull/3578


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>