fix end function when piping on request

lib/test.js

@@ -109,7 +109,7 @@ class Test extends Request {
    * Defer invoking superagent's `.end()` until
    * the server is listening.
    *
-   * @param {Function} fn
+   * @param {?Function} fn
    * @api public
    */
   end(fn) {
@@ -133,7 +133,7 @@ class Test extends Request {
    *
    * @param {?Error} resError
    * @param {Response} res
-   * @param {Function} fn
+   * @param {?Function} fn
    * @api private
    */
   assert(resError, res, fn) {
@@ -169,7 +169,9 @@ class Test extends Request {
       errorObj = resError;
     }
 
-    fn.call(this, errorObj || null, res);
+    if (fn) {
+      fn.call(this, errorObj || null, res);
+    }
   }
 
   /**

test/supertest.js

@@ -9,6 +9,7 @@ try {
 }
 const fs = require('fs');
 const path = require('path');
+const { Readable } = require('stream');
 const should = require('should');
 const express = require('express');
 const bodyParser = require('body-parser');
@@ -155,6 +156,28 @@ describe('request(app)', function () {
       .expect('john', done);
   });
 
+  it('should work with pipe', function (done) {
+    const app = express();
+
+    app.use(express.json());
+
+    app.post('/', function (req, res) {
+      res.send(req.body.name);
+    });
+
+    const body = Readable.from(JSON.stringify({ name: 'john' }));
+    const req = request(app)
+      .post('/')
+      .set('Content-Type', 'application/json')
+      .on('response', function (res) {
+        should.exist(res);
+        res.status.should.be.equal(200);
+        res.text.should.be.equal('john');
+        done();
+      });
+    body.pipe(req);
+  });
+
   it('should work when unbuffered', function (done) {
     const app = express();