http: fix incorrect headersTimeout measurement

For keep-alive connections, the headersTimeout may fire during
subsequent request because the measurement was reset after
a request and not before a request.

PR-URL: https://github.com/nodejs/node/pull/32329
Fixes: https://github.com/nodejs/node/issues/27363


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>