title: await page.cookies() not returning cookies seen in response header
body: <p>I am executing a basic puppeteer script that opens a webpage looks at response values and then pulls the cookies.</p>
  
  <p>In the example below the response header has the following key values:</p>
  
  <pre><code>page.on('response', response =&gt; {
    const req = response.request();
    const resp = response;
    let result = {};
    result['method'] = req.method;
    result['status'] = resp.status;
    result['url'] = req.url;
    result['headers'] = resp.headers;
  
  
  // Output of result['headers'] shortened for example
       u'server': u'Microsoft-IIS/7.5',
       u'set-cookie': u'A6=030uxloava000EG.000010000; expires=Mon, 16-Apr-2018 21:30:52 GMT; domain=.serving-sys.com; path=/\nC6=; expires=Mon, 16-Apr-2018 21:30:52 GMT; domain=.serving-sys.com; path=/\nD3=; expires=Mon, 16-Apr-2018 21:30:52 GMT; domain=.serving-sys.com; path=/\nu2=7f11f3f6-8979-4adc-824e-4d43b67f9b374ib310; expires=Mon, 16-Apr-2018 21:30:52 GMT; domain=.serving-sys.com; path=/',
       u'x-powered-by': u'ASP.NET'},
  </code></pre>
  
  <p>In that header you'll see a set-cookie but when I call await page.cookies() right after page.on('response'... :</p>
  
  <pre><code>    const cookies = await page.cookies();
  </code></pre>
  
  <p>const cookies will be []. Am I missing something here?</p>