http2: add compat support for nested array headers

writeHead supports an array of arrays containing header name and values.
Compatibility between http2 & http1 even though this is not documented.

Fixes: https://github.com/nodejs/node/issues/24466

PR-URL: https://github.com/nodejs/node/pull/24665


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>