s3: fix KS3 problem where multipart uploads have valid md5sums as Etags

KS3 appears to return an Etag which is a valid MD5SUM for multipart
uploads. This confuses rclone which is expecting an invalid MD5SUM
here.

This patch works around that by clearing the Etag on the object
returned from a multipart upload if it matches a valid MD5SUM.

backend/s3/s3.go

@@ -3202,6 +3202,12 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	// Read the metadata from the newly created object
 	o.meta = nil // wipe old metadata
 	err = o.readMetaData(ctx)
+	// Empty an Etag which is a valid md5sum for multipart
+	// uploads. This works around a bug in KS3 where the ETag is a
+	// correctly formed md5sum for multpart uploads
+	if multipart && matchMd5.MatchString(strings.Trim(strings.ToLower(o.etag), `"`)) {
+		o.etag = ""
+	}
 	return err
 }