Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'url') and self.url is not None:
            _dict['url'] = self.url
        if hasattr(self, 'limit_to_starting_hosts'
                  ) and self.limit_to_starting_hosts is not None:
            _dict['limit_to_starting_hosts'] = self.limit_to_starting_hosts
        if hasattr(self, 'crawl_speed') and self.crawl_speed is not None:
            _dict['crawl_speed'] = self.crawl_speed
        if hasattr(self, 'allow_untrusted_certificate'
                  ) and self.allow_untrusted_certificate is not None:
            _dict[
                'allow_untrusted_certificate'] = self.allow_untrusted_certificate
        if hasattr(self, 'maximum_hops') and self.maximum_hops is not None:
            _dict['maximum_hops'] = self.maximum_hops
        if hasattr(self,
                   'request_timeout') and self.request_timeout is not None:
            _dict['request_timeout'] = self.request_timeout
        if hasattr(
                self,
                'override_robots_txt') and self.override_robots_txt is not None:
            _dict['override_robots_txt'] = self.override_robots_txt
        return _dict