touch the config on save, even if no changes were made

Sometimes, the DokuWiki's cache needs to be invalidated. The simplest way to do so is touching the local.php. However this is complicated for Windows and FTP-Users. One workaround is to change something in the config, save, change it back, save again. Still a bit complicated. This is because the config will not be saved when nothing was changed. This patch will simply touch the config when on saving, when no change was made.