title: ProxyError only on post requests, not on get
body: <pre><code>import requests
  
  s = requests.session()
  s.proxies = {'https': 'https://xxx.xxx.xxx.xxx:8888'}
  s.get("https://www.tumblr.com/")
  s.post("https://www.tumblr.com/")
  </code></pre>
  
  <p>GET will work fine, but POST will throw a ProxyError. Any ideas why?</p>
  
  <blockquote>
    <p>ProxyError: Cannot connect to proxy. Socket error: Tunnel connection
    failed: 400 Bad Request.</p>
  </blockquote>