docs: def _default_headers(self):
          """Set the default header for a Twilio SendGrid v3 API call"""
          headers = {
              "Authorization": 'Bearer {}'.format(self.api_key),
              "User-agent": self.useragent,
              "Accept": 'application/json'
          }
          if self.impersonate_subuser:
              headers['On-Behalf-Of'] = self.impersonate_subuser
  
          return headers