buffer middleware still attempt the request even though client cancel it

[onestraw]

httputil.ReverseProxy will return 502 BadGateway error if client cancel the request.

buffer middleware will retry if we configured retry with IsNetworkError() which capture 502 code.

If this 502 code is caused by client canceled request, it's not necessary to retry.

BTW, Nginx will return it's self-defined code 499 in this case.