fix issue with product type nested set options

Fixes this error I have been having.
```
ActionView::Template::Error (no implicit conversion from nil to integer)
```

> As documented on the gem's page, the block passed to the `nested_set_options` only accepts a single argument `i` which is the category itself. level is a method of `i` which you can get using `i.level`.
> -http://stackoverflow.com/a/20139429/1417774