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

benchmark/_http-benchmarkers.js

@@ -64,6 +64,9 @@ class WrkBenchmarker {
       '-t', 8,
       `http://127.0.0.1:${options.port}${options.path}`,
     ];
+    for (const field in options.headers) {
+      args.push('-H', `${field}: ${options.headers[field]}`);
+    }
     const child = child_process.spawn(this.executable, args);
     return child;
   }