http: don't cork noop .end()

Calling .end() a second time should be a noop and not
leave the socket corked.

Fixes: https://github.com/nodejs/node/issues/36620

PR-URL: https://github.com/nodejs/node/pull/36633


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>