the supertest.agent function can not run correctly

I use the version of 6.2.3, and wanna use agent function to save cookie to client side. But the code below can not run correctly:
```javascript
const request = require('supertest');

describe('agent test', function () {
it('get agent', function (done) {
const agent = request.agent();
console.log(agent);
agent.get('http://bing.com').end(function (err, res) {
if (err) {
return done(err);
}
console.log(agent);
});
});
});
```
It will end with such error:
```
TypeError: Cannot read property 'address' of undefined
at Test.serverAddress (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_supertest@6.2.3@supertest\lib\test.js:46:22)
at new Test (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_supertest@6.2.3@supertest\lib\test.js:34:14)
at TestAgent. [as get] (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_supertest@6.2.3@supertest\lib\agent.js:47:17)
at Context. (E:\kuaipan\code\node2\myapp\chapter8\src\test\http\agent.test.js:7:15)
at callFnAsync (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runnable.js:400:21)
at Test.Runnable.run (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runnable.js:342:7)
at Runner.runTest (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:455:10)
at E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:573:12
at next (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:369:14)
at E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:379:7
at next (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:303:14)
at Immediate._onImmediate (E:\kuaipan\code\node2\myapp\chapter8\node_modules\_mocha@5.2.0@mocha\lib\runner.js:347:5)
at processImmediate (internal/timers.js:464:21)
at process.callbackTrampoline (internal/async_hooks.js:130:17)
```