docs: def clean_headers(self):
          """Only clean the headers (cookie include) and return self."""
          if not isinstance(self.request.get('headers'), dict):
              return self
          headers = self.request['headers']
          if 'Cookie' in headers:
              self.clean_cookie()
          for key in headers:
              # cookie will be checked in other methods.
              if key == 'Cookie':
                  continue
              new_request = deepcopy(self.request)
              new_headers = deepcopy(headers)
              new_headers.pop(key)
              new_request['headers'] = new_headers
              self._add_task('headers', key, new_request)
          return self