Fixed response code check in Result class

testsuite/httpx/__init__.py

@@ -30,7 +30,7 @@ def __init__(self, retry_codes, response=None, error=None):
 
     def should_backoff(self):
         """True, if the Result can be considered an instability and should be retried"""
-        return self.has_dns_error() or (self.error is None and self.status_code in self.retry_codes)
+        return self.has_dns_error() or (self.error is None and self.response.status_code in self.retry_codes)
 
     def has_error(self, error_msg: str) -> bool:
         """True, if the request failed and an error with message was returned"""