http: use more efficient module.exports pattern

PR-URL: https://github.com/nodejs/node/pull/11594


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>