http: check statusCode early

By enforcing the statusCode to be an SMI, it helps a bit
performance-wise when looking up the associated statusMessage.

PR-URL: https://github.com/nodejs/node/pull/10558


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>