fix end function when piping on request

[lafeuil]

When I want to pipe to the request :

    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);

I have an error Uncaught TypeError: Cannot read property 'call' of undefined

This pull request have :

• a unit test to reproduce the issue
• the small fix that removes the call to callback function in the end function if it is empty.

[niftylettuce]

Have you tried again with latest version?