06-content-negotiation, "content-type" required when encoding is "gzip"

When accept-encoding is gzip, we need to set the content-type to text/plain.

this.response.set('Content-Encoding', 'gzip');
this.response.body = yield gzip('hello world');

The above code would fail the test because, when content-enconding is gzip, the default content-type is application/octet-stream.

Adding the content-type assignment would solve the problem.

this.response.set('Content-Encoding', 'gzip');
this.response.set('content-type', 'text/plain');
this.response.body = yield gzip('hello world');

app.use(function* () {
  const ae = this.request.acceptsEncodings('gzip', 'identity');

  switch (ae) {
    case 'gzip':
      this.response.set('Content-Encoding', 'gzip');
      this.response.set('content-type', 'text/plain');
      this.response.body = yield gzip('hello world');
      break;
    case 'identity':
      this.response.set('Content-Encoding', 'identity');
      this.response.body = 'hello world';
      break;
    default:
      break;
  }
});

Start with the chapter 06 content-negotiation example and the test that fails for a gzip response. Verify the response handling shown in the issue, then ensure the gzip case declares text/plain as its content type and the test passes.