http: fix http agent keep alive

lib/_http_agent.js

@@ -342,7 +342,11 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
     installListeners(this, s, options);
     cb(null, s);
   });
-
+  // When keepAlive is true, pass the related options to createConnection
+  if (this.keepAlive) {
+    options.keepAlive = this.keepAlive;
+    options.keepAliveInitialDelay = this.keepAliveMsecs;
+  }
   const newSocket = this.createConnection(options, oncreate);
   if (newSocket)
     oncreate(null, newSocket);

test/parallel/test-http-agent-keepalive-delay.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const { Agent } = require('_http_agent');
+
+const agent = new Agent({
+  keepAlive: true,
+  keepAliveMsecs: 1000,
+});
+
+const server = http.createServer(common.mustCall((req, res) => {
+  res.end('ok');
+}));
+
+server.listen(0, common.mustCall(() => {
+  const createConnection = agent.createConnection;
+  agent.createConnection = (options, ...args) => {
+    assert.strictEqual(options.keepAlive, true);
+    assert.strictEqual(options.keepAliveInitialDelay, agent.keepAliveMsecs);
+    return createConnection.call(agent, options, ...args);
+  };
+  http.get({
+    host: 'localhost',
+    port: server.address().port,
+    agent: agent,
+    path: '/'
+  }, common.mustCall((res) => {
+    // for emit end event
+    res.on('data', () => {});
+    res.on('end', () => {
+      server.close();
+    });
+  }));
+}));