http: allow passing array of key/val into writeHead

Enables an optimization when the user already has the headers
in an array form, e.g. when proxying.

lib/_http_outgoing.js

@@ -381,8 +381,14 @@ function _storeHeader(firstLine, headers) {
         processHeader(this, state, entry[0], entry[1], false);
       }
     } else if (ArrayIsArray(headers)) {
-      for (const entry of headers) {
-        processHeader(this, state, entry[0], entry[1], true);
+      if (headers.length && ArrayIsArray(headers[0])) {
+        for (const entry of headers) {
+          processHeader(this, state, entry[0], entry[1], true);
+        }
+      } else {
+        for (let n = 0; n < headers.length; n += 2) {
+          processHeader(this, state, headers[n + 0], headers[n + 1], true);
+        }
       }
     } else {
       for (const key in headers) {

lib/_http_server.js

@@ -22,6 +22,7 @@
 'use strict';
 
 const {
+  ArrayIsArray,
   Error,
   ObjectKeys,
   ObjectSetPrototypeOf,
@@ -278,7 +279,12 @@ function writeHead(statusCode, reason, obj) {
   if (this[kOutHeaders]) {
     // Slow-case: when progressive API and header fields are passed.
     let k;
-    if (obj) {
+    if (ArrayIsArray(obj)) {
+      for (let n = 0; n < obj.length; n += 2) {
+        k = obj[n + 0];
+        if (k) this.setHeader(k, obj[n + 1]);
+      }
+    } else if (obj) {
       const keys = ObjectKeys(obj);
       // Retain for(;;) loop for performance reasons
       // Refs: https://github.com/nodejs/node/pull/30958

test/parallel/test-http-write-head-2.js

@@ -0,0 +1,41 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// Verify that ServerResponse.writeHead() works with arrays.
+
+{
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.setHeader('test', '1');
+    res.writeHead(200, [ 'test', '2', 'test2', '2' ]);
+    res.end();
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    http.get({ port: server.address().port }, common.mustCall((res) => {
+      assert.strictEqual(res.headers.test, '2');
+      assert.strictEqual(res.headers.test2, '2');
+      res.resume().on('end', () => {
+        server.close();
+      });
+    }));
+  }));
+}
+
+{
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, [ 'test', '1', 'test2', '2' ]);
+    res.end();
+  }));
+
+  server.listen(0, common.mustCall(function() {
+    http.get({ port: server.address().port }, common.mustCall((res) => {
+      assert.strictEqual(res.headers.test, '1');
+      assert.strictEqual(res.headers.test2, '2');
+      res.resume().on('end', () => {
+        server.close();
+      });
+    }));
+  }));
+}