title: pyppeteer setCookie issue
body: <p>When I use pyppeteer with cookies, the below exception is raised</p>
  
  <blockquote>
    <p>File "test.py", line 33, in 
        asyncio.get_event_loop().run_until_complete(main())</p>
    
    <p>File "/usr/local/python3.6/lib/python3.6/asyncio/base_events.py", line 468,
    in run_until_complete
        return future.result()</p>
    
    <p>File "test.py", line 25, in main
        await page.setCookie(cookies)</p>
    
    <p>File "/usr/local/python3.6/lib/python3.6/site-packages/pyppeteer/page.py",
    line 392, in setCookie
        await self.deleteCookie(*items)</p>
    
    <p>File "/usr/local/python3.6/lib/python3.6/site-packages/pyppeteer/page.py",
    line 373, in deleteCookie
        await self._client.send('Network.deleteCookies', item)</p>
    
    <p>File "/usr/local/python3.6/lib/python3.6/site-packages/pyppeteer/connection.py",
    line 200, in send
        return await callback </p>
    
    <p>pyppeteer.errors.NetworkError: Protocol Error: Invalid parameters name: string value expected</p>
  </blockquote>
  
  <p>My code listed:</p>
  
  <pre><code>async def main():
      cookies = {'name': "", 'name2': ""}  # just a example
      browser = await launch({"executablePath": "/usr/bin/google-chrome-stable"}, args=['--no-sandbox'], headless=True)
      page = await browser.newPage()
      await page.setCookie(cookies)
      print('after set cookie')
      response = await page.goto('http://example.com/', timeout=100000)
  
      print('before screenshot')
      await page.screenshot({'path': '/tmp/tmp.png', 'quality': 100, 'fullPage': True})
      await browser.close()
  
  asyncio.get_event_loop().run_until_complete(main())
  </code></pre>