https: support agent construction without new

Fixes: https://github.com/nodejs/node/issues/12918
PR-URL: https://github.com/nodejs/node/pull/12927
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/https.js

@@ -97,6 +97,9 @@ function createConnection(port, host, options) {
 
 
 function Agent(options) {
+  if (!(this instanceof Agent))
+    return new Agent(options);
+
   http.Agent.call(this, options);
   this.defaultPort = 443;
   this.protocol = 'https:';

test/parallel/test-https-agent-constructor.js

@@ -0,0 +1,7 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const https = require('https');
+
+assert.doesNotThrow(() => { https.Agent(); });
+assert.ok(https.Agent() instanceof https.Agent);