fix: makes object metadata keys lowercase in object creation actions

Fixes #1482

The metadata keys should always be converted to lowercase in `PutObject`, `CreateMultipartUpload`, and `CopyObject`. This implementation converts the metadata keys to lowercase in the front end, ensuring they are stored in lowercase in the backend.

s3api/utils/utils.go

@@ -58,7 +58,7 @@ func GetUserMetaData(headers *fasthttp.RequestHeader) (metadata map[string]strin
 	for key, value := range headers.AllInOrder() {
 		hKey := string(key)
 		if strings.HasPrefix(strings.ToLower(hKey), "x-amz-meta-") {
-			trimmedKey := hKey[11:]
+			trimmedKey := strings.ToLower(hKey[11:])
 			headerValue := string(value)
 			metadata[trimmedKey] = headerValue
 		}