http: improve validation performance

The new table-based lookups perform significantly better for the
common cases (checking latin1 characters).

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>