`oo7::Item::set_attributes()` with the file backend creates a new item

[zecakeh]

The reason to call that method should be to update the attributes of an existing item.

However, for the file backend, that method calls Keyring::create_item() with replace: true, that only replaces items with the same attributes, so if the attributes changed (which is usually the case) a new item is created.

Maybe the implementation should be changed to use Keyring::replace_item_index() instead?