title: puppeteer page.authenticate https proxy not work
body: <p>Running puppeteer with Proxy authentication failing.</p>
  
  <p>Puppeteer version: 1.8</p>
  
  <p>Platform / OS version:MacOS 10.13.6</p>
  
  <p>Node.js version:v10.9.0</p>
  
  <pre><code>    const puppeteer = require('puppeteer');
  
      (async () =&gt; {
          const browser = await puppeteer.launch({
          headless:false,
          ignoreHTTPSErrors:true,
          devtools:true,
          timeout:3000,
          args: ['--no-sandbox','--proxy-server=xxx:xxx']
       });
       const user='xxx';
       const password='xxx';
       const page = await browser.newPage();
       // await page.setExtraHTTPHeaders({
       //   'Proxy-Authorization': 'Basic ' + Buffer.from(`${user}:${password}`).toString('base64'),
     // });
       await page.authenticate({username:user, password:password});
       await page.goto('https://www.apple.com/');
       let title = await page.title();
       console.log("title:" + title);
       await browser.close();
      })();
  </code></pre>
  
  <p>got error:</p>
  
  <pre><code>(node:5858) UnhandledPromiseRejectionWarning: 
  Error: net::ERR_TUNNEL_CONNECTION_FAILED at https://www.apple.com/
  at navigate (/xxx/node_modules/_puppeteer@1.8.0@puppeteer/lib/Page.js:622:37)
  at process._tickCallback (internal/process/next_tick.js:68:7)
  (node:5858) UnhandledPromiseRejectionWarning:
  Unhandled promise rejection. This error originated either by throwing
  inside of an async function without a catch block, 
  or by rejecting a promise which was not handled with .catch(). 
  (rejection id: 1)
  </code></pre>