title: Getting error while trying to add header with Set-Cookie in GAE
body: <p>I am trying to include external python module in my project for working with sessions. It's named <code>gmemsess.py</code>. It tries to add Set-Cookie header in the response and an error appears:</p>
  
  <pre><code>rh.response.headers.add_header('Set-Cookie','%s=%s; path=/;'%(name,self._sid))
  AttributeError: HeaderDict instance has no attribute 'add_header'
  </code></pre>
  
  <p>I read documentation and everything seems ok, but it doesn't work. Why this error can appear?
  Also, I use webapp2 to manage subdomains. May be there is something goes wrong because of this?</p>