http: optimize headers iteration

This commit uses instanceof instead of Array.isArray() for faster
type checking and avoids calling Object.keys() when the headers are
stored as a 2D array instead of a plain object.

PR-URL: https://github.com/nodejs/node/pull/6533


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>