http: make socketPath work with no agent

Currently `Agent.prototype.createConnection()` is called uncoditionally
if the `socketPath` option is used. This throws an error if no agent is
used, preventing, for example, the `socketPath` and `createConnection`
options to be used together.

This commit fixes the issue by falling back to the `createConnection`
option or `net.createConnection()`.

lib/_http_client.js

@@ -138,7 +138,11 @@ function ClientRequest(options, cb) {
       timeout: self.timeout,
       rejectUnauthorized: !!options.rejectUnauthorized
     };
-    const newSocket = self.agent.createConnection(optionsPath, oncreate);
+    const newSocket = self.agent
+      ? self.agent.createConnection(optionsPath, oncreate)
+      : typeof options.createConnection === 'function'
+        ? options.createConnection(optionsPath, oncreate)
+        : net.createConnection(optionsPath);
     if (newSocket && !called) {
       called = true;
       self.onSocket(newSocket);

test/parallel/test-http-client-unix-socket-no-agent.js

@@ -0,0 +1,28 @@
+'use strict';
+const common = require('../common');
+const Countdown = require('../common/countdown');
+
+const http = require('http');
+const { createConnection } = require('net');
+
+const server = http.createServer((req, res) => {
+  res.end();
+});
+
+const countdown = new Countdown(2, () => {
+  server.close();
+});
+
+common.refreshTmpDir();
+
+server.listen(common.PIPE, common.mustCall(() => {
+  http.get({ createConnection, socketPath: common.PIPE }, onResponse);
+  http.get({ agent: 0, socketPath: common.PIPE }, onResponse);
+}));
+
+function onResponse(res) {
+  res.on('end', () => {
+    countdown.dec();
+  });
+  res.resume();
+}