http: append Cookie header values with semicolon

Previously, separate incoming Cookie headers would be concatenated
with a comma, which can cause confusion in userland code when it
comes to parsing the final Cookie header value. This commit
concatenates using a semicolon instead.

Fixes: https://github.com/nodejs/node/issues/11256
PR-URL: https://github.com/nodejs/node/pull/11259


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>