fix (s3): mv does not work for nested objects

The mv operation executes CopyObject API on the root object, meanwhile
CopyObject doesn't copy the nested objects.

As a result, the user loses all nested objects and after the move
operation gets an empty bucket that requested to move.

This change disallows possibility of moving nested objects.

server/model/backend/s3.go

@@ -275,7 +275,7 @@ func (s S3Backend) Mv(from string, to string) error {
 	t := s.path(to)
 	client := s3.New(s.createSession(f.bucket))
 
-	if f.path == "" {
+	if f.path == "" || strings.HasSuffix(from, "/") {
 		return NewError("Can't move this", 403)
 	}