code: def remember(self, request, username, **kw):
          
          if self.credentials_callback:
              token = self.credentials_callback(username, request)
              api_key = 'ApiKey {}:{}'.format(username, token)
              return [('WWW-Authenticate', api_key)]
docs: Returns 'WWW-Authenticate' header with a value that should be used
          in 'Authorization' header.