fix: multipart replication and encrypted etag for sse-s3 (#13171)

Replication was not working properly for encrypted
objects in single PUT object for preserving etag,

We need to make sure to preserve etag such that replication
works properly and not gets into infinite loops of copying
due to ETag mismatches.

cmd/object-api-utils.go

@@ -290,10 +290,10 @@ func cleanMetadataKeys(metadata map[string]string, keyNames ...string) map[strin
 
 // Extracts etag value from the metadata.
 func extractETag(metadata map[string]string) string {
-	// md5Sum tag is kept for backward compatibility.
-	etag, ok := metadata["md5Sum"]
+	etag, ok := metadata["etag"]
 	if !ok {
-		etag = metadata["etag"]
+		// md5Sum tag is kept for backward compatibility.
+		etag = metadata["md5Sum"]
 	}
 	// Success.
 	return etag