title: Python requests session does not rotate proxies
body: <p>I am using private rotating proxy provided by (<a href="https://proxy.webshare.io/proxy/rotating?" rel="nofollow noreferrer">https://proxy.webshare.io/proxy/rotating?</a>) in which each request to rotating proxy receives a new IP address. when I am using
  <code>requests.get('https://httpbin.org/get', headers=headers, proxies=get_proxy())</code>
  it returns a new IP each time whenever I make request. but when using</p>
  <pre><code>session = requests.Session()
  session.headers = headers
  session.proxies = get_proxy()
  
  session.get('https://httpbin.org/get')
  </code></pre>
  <p>It returns same IP each time whenever I make request.
  How does session object behaves different from requests.get() function in case of proxies.</p>