Fix "can't modify frozen string" error in validation callbacks
Fix "can't modify frozen string" error in validation callbacks

lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,mkllnk/openfoodnetwork,Matt-Yorkley/openfoodnetwork,mkllnk/openfoodnetwork,Matt-Yorkley/openfoodnetwork,Matt-Yorkley/openfoodnetwork,mkllnk/openfoodnetwork,openfoodfoundation/openfoodnetwork,mkllnk/openfoodnetwork,lin-d-hop/openfoodnetwork,Matt-Yorkley/openfoodnetwork,openfoodfoundation/openfoodnetwork,lin-d-hop/openfoodnetwork