docs: def x_forwarded_for(self):
          """X-Forwarded-For header value.
  
          This is the amended header so that it contains the previous IP address
          in the forwarding change.
  
          """
          ip = self._request.META.get('REMOTE_ADDR')
          current_xff = self.headers.get('X-Forwarded-For')
  
          return '%s, %s' % (current_xff, ip) if current_xff else ip