Use product_type slug not sanitized term name

As mentioned [here](https://github.com/woothemes/woocommerce/commit/b2a868301c622d03702f58285cb6d97972511953#commitcomment-2663540), if there is no reason `WC_Product_Factory->get_product() needs to use `sanitize_title( current( $terms )->name )` instead of `current( $terms )->slug`, then I think this is a better options - slug is effectively a pre-sanitised name and it can be set when creating the term to allow for custom class names which:

1. can have class names conforming to coding standards - e.g. `WC_Product_Variable_Subscription` rather than `WC_Product_Variable-subscription`
1. do not require use of the `woocommerce_product_class` filter