http: Corrects IPv6 address in Host header

IPv6 addresses in Host header (URI), must be enclosed within
square brackets, in order to properly separate the host address
from any port reference.

PR-URL: https://github.com/nodejs/node/pull/5314


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