move tolower inside if

Author: yousukseung

xds/internal/balancer/ringhash/config.go

@@ -72,8 +72,8 @@ func parseConfig(c json.RawMessage) (*LBConfig, error) {
 	if !envconfig.RingHashSetRequestHashKey {
 		cfg.RequestHashHeader = ""
 	}
-	cfg.RequestHashHeader = strings.ToLower(cfg.RequestHashHeader)
 	if cfg.RequestHashHeader != "" {
+		cfg.RequestHashHeader = strings.ToLower(cfg.RequestHashHeader)
 		// See rules in https://github.com/grpc/proposal/blob/master/A76-ring-hash-improvements.md#explicitly-setting-the-request-hash-key
 		if err := metadata.ValidateKey(cfg.RequestHashHeader); err != nil {
 			return nil, fmt.Errorf("invalid requestHashHeader %q: %v", cfg.RequestHashHeader, err)