title: Scrapy cookies not working when sending Proxy-Authorization header
body: <p>I'm using proxy like this:</p>
  
  <pre><code>request = Request(url="www.domain.com")    
  
  in middleware:
  
  request.meta['proxy'] = "http://2.2.2.2:8000"
  user_pass = base64.encodestring('username:password')
  request.headers['Proxy-Authorization'] = 'Basic ' + user_pass
  </code></pre>
  
  <p>And cookies like this:</p>
  
  <pre><code>request = Request(url="www.domain.com", cookies={'preferences': 'ps=www2'})
  </code></pre>
  
  <p>When I using cookies and proxy separately all works fine, but when I'm trying to combine both cookies and proxy in one request:</p>
  
  <pre><code>request = Request(url="www.domain.com", cookies={'preferences': 'ps=www2'})
  
  in middleware:
  
  request.meta['proxy'] = "http://2.2.2.2:8000"
  user_pass = base64.encodestring('username:password')
  request.headers['Proxy-Authorization'] = 'Basic ' + user_pass
  </code></pre>
  
  <p>cookies are just not sent to the server.</p>
  
  <p>My suggestion: something wrong with sending header for proxy authorization. It's just cutting off cookies.</p>