Fix numeric folders throwing on markDirty

TypeError: strpos() expects parameter 1 to be string, int given

The problem is that in cacheNode() we strip of any slashes, so
a folder "0/" will be trimmed to "0" and be used as an array key.
Since PHP automatically casts numeric array keys to integers,
you afterwards get $nodePath as int(0). Since it's now a number,
the strpos() function does not accept it anymore. Simply casting
$nodePath to a string again in the foreach solves the issue

Signed-off-by: Joas Schilling <coding@schilljs.com>