http2: store headersSent after stream destroyed

Store headersSent directly on response state after finish event
is triggered, so that users can always access it.

PR-URL: https://github.com/nodejs/node/pull/15232
Fixes: https://github.com/nodejs/node/issues/15226


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>