http2: pass incoming set-cookie header as array

Incoming set-cookie headers should be passed to user as array like in
http module.

Besides improving compatibility between http and http2 it avoids the
need to check if the type is an array or not in user code.

PR-URL: https://github.com/nodejs/node/pull/21360


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>