Suggestion to fix the issue #4495

The WC_Settings_Products class generates a [special case](https://github.com/woothemes/woocommerce/blob/master/includes/admin/settings/class-wc-settings-products.php#L68-L75) to show the options of products and inventory.
With that the inventory options are not saved on WooCommerce installation, because of this I could only think of a special condition for the installation of the inventory.

More details in #4495.