code: def headerlist(self):
          
          if 'Content-Type' not in self.headers:
              self.headers.add_header('Content-Type', self.default_content_type)
          if self._cookies:
              for c in self._cookies.values():
                  self.headers.add_header('Set-Cookie', c.OutputString())
          return self.headers.items()
docs: WSGI conform list of (header, value) tuples.