remove some http hard code (#2047)

Co-authored-by: yonbiaoxiao <yonbiaoxiao@tencent.com>

client/retry.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"net/http"
 
 	"github.com/micro/go-micro/v3/errors"
 )
@@ -27,7 +28,7 @@ func RetryOnError(ctx context.Context, req Request, retryCount int, err error) (
 
 	switch e.Code {
 	// retry on timeout or internal server error
-	case 408, 500:
+	case http.StatusRequestTimeout, http.StatusInternalServerError:
 		return true, nil
 	default:
 		return false, nil