fix: set user metadata key to lowercase (#3894)

* fix: set user metadata key to lowercase

* feat: simplify a bit

weed/s3api/s3api_object_handlers.go

@@ -409,10 +409,22 @@ func (s3a *S3ApiServer) proxyToFiler(w http.ResponseWriter, r *http.Request, des
 		return
 	}
 
+	setUserMetadataKeyToLowercase(resp)
+
 	responseStatusCode := responseFn(resp, w)
 	s3err.PostLog(r, responseStatusCode, s3err.ErrNone)
 }
 
+func setUserMetadataKeyToLowercase(resp *http.Response) {
+	for key, value := range resp.Header {
+		if strings.HasPrefix(key, s3_constants.AmzUserMetaPrefix) {
+			resp.Header[strings.ToLower(key)] = value
+			delete(resp.Header,key)
+			continue
+		}
+	}
+}
+
 func passThroughResponse(proxyResponse *http.Response, w http.ResponseWriter) (statusCode int) {
 	for k, v := range proxyResponse.Header {
 		w.Header()[k] = v