https: make https.globalAgent overridable also under ECMAScript Modules

Under ECMAScript modules when you do "import * as https from 'https'"
you get a new object with properties copied from https module exports.
So if this is a regular data property, you will just override a copy,
but if this would be a accessor property, we can still access the actual
https.globalAgent.

Refs: nodejs#25170, nodejs#9386

lib/https.js

@@ -350,7 +350,7 @@ Agent.prototype._evictSession = function _evictSession(key) {
   delete this._sessionCache.map[key];
 };
 
-const globalAgent = new Agent({ keepAlive: true, scheduling: 'lifo', timeout: 5000 });
+let globalAgent = new Agent({ keepAlive: true, scheduling: 'lifo', timeout: 5000 });
 
 /**
  * Makes a request to a secure web server.
@@ -415,9 +415,21 @@ function get(input, options, cb) {
 
 module.exports = {
   Agent,
-  globalAgent,
   Server,
   createServer,
   get,
   request,
 };
+
+// make https.globalAgent overridable also under ECMAScript Modules
+ObjectDefineProperty(module.exports, 'globalAgent', {
+  __proto__: null,
+  configurable: true,
+  enumerable: true,
+  get() {
+    return globalAgent;
+  },
+  set(value) {
+    globalAgent = value;
+  },
+});