benchmark: support optional headers with wrk

wrk did not support sending optional headers, they were silently
ignored. This appears to be an oversight, they were supported for
autocannon.

PR-URL: https://github.com/nodejs-private/node-private/pull/189


Reviewed-By: Matteo <Collina &lt;matteo.collina@gmail.com>
Reviewed-By: Fedor <Indutny &lt;fedor.indutny@gmail.com>
Reviewed-By: Ben <Noordhuis &lt;info@bnoordhuis.nl>
Reviewed-By: James M <Snell &lt;jasnell@gmail.com>