access.ftp fails to check if files are writable

I've found that, to check if a repository file is writable, access.ftp seems to check if web server user can write to that file. But this check is wrong for FTP repositories, it should check if FTP user can write to that file, not web server user.
Thus, delete/rename/etc. buttons are only enabled when files are owned by web server user (www-data or whatever) and not when they belong to FTP user currently logged in.
May it be related to issue #429? I already tried the patch there with no success.