title: python-requests - user-agent is being overriden
body: <p>I have</p>
  
  <pre><code>    logindata = {
          'username': 'me',
          'password': 'blbla'
      }
      payload = {'from':'me', 'lang':'en', 'url':csv_url}
      headers = {
          'User-Agent': 'Mozilla/5.0'
      }
      api_url = 'http://dev.mypage.com/admin/app/import/'
  
      with requests.Session() as s:
          s.post(api_url, data=json.dumps(logindata), headers=headers)
  
          print s.headers
  
          # An authorised request.
          r = s.get(api_url, params=payload, headers=headers)
  </code></pre>
  
  <p>I am getting rejected but it is because of 403 forbidden. And I printed the headers, I get: </p>
  
  <pre><code>..'User-Agent': 'python-requests/2.2.1 CPython/2.7.5 Windows/7'..
  </code></pre>
  
  <p>Why is my <code>'User-Agent': 'Mozilla/5.0'</code> getting overriden? what am I missing here? </p>