docs: def copy(self, cls=None):
          ''' Returns a copy of self. '''
          cls = cls or BaseResponse
          assert issubclass(cls, BaseResponse)
          copy = cls()
          copy.status = self.status
          copy._headers = dict((k, v[:]) for (k, v) in self._headers.items())
          if self._cookies:
              copy._cookies = SimpleCookie()
              copy._cookies.load(self._cookies.output(header=''))
          return copy