http: concatenate outgoing Cookie headers

This commit enables automatic concatenation of multiple Cookie header
values with a semicolon, except when 2D header arrays are used.

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>