docs: def authorize_url(self, state=''):
          """ return user authorize url
          """
          url = 'https://openapi.youku.com/v2/oauth2/authorize?'
          params = {
              'client_id': self.client_id,
              'response_type': 'code',
              'state': state,
              'redirect_uri': self.redirect_uri
          }
          return url + urlencode(params)