Update ProductMappingExtensions.cs

Don't know if this code is correct (copied more or less from ToProduct()).
Needs some extra testing...

If you want to ship a variant, you get these errors when trying to get ShipmentRateQuotes(): "No CatalogKeys found in Shipment Items" and "ProductVariant marked as shippable was not assoicated with a WarehouseCatalog".

The problem is, that the checkbox for the Warehouse is not saved in the product variant editor.