Keep product auto_item unused in AbstractProduct::getCssClass()

[fritzmg]

The ProductList retrieves each individual product's CSS class here:

core/system/modules/isotope/library/Isotope/Module/ProductList.php

Line 281 in d4c91ed

'class' => $objProduct->getCssClass(),

This causes invalid auto_item URLs on the product list page to become valid (i.e. they will not show 404). For example if you have a product list on

https://example.com/shop

then the invalid URL

https://example.com/shop/not-a-valid-url-parameter

will not show the 404 page. Instead it will just show the contents of the https://example.com/shop page.

This PR fixes that by enabling the $blnKeepUnused parameter.

[aschempp]

Thank you @fritzmg