code: def get_single_header(headers, key):
      
      raw_headers = headers.getRawHeaders(key)
      if raw_headers is None:
          return None
  
      # Take the final header as the authorative
      header, _ = cgi.parse_header(raw_headers[-1])
      return header
docs: Get a single value for the given key out of the given set of headers.
  
      :param twisted.web.http_headers.Headers headers:
          The set of headers in which to look for the header value
      :param str key:
          The header key