Fulfillments: Gift Message Inherits From The Order

A fulfillment's gift message now falls back to the order's gift message whenever the fulfillment doesn't have one of its own. Setting a gift message on the fulfillment still overrides the order, same as before. Outbound payloads to ShipStation, ShipHero, Packiyo, MasonHub, and Infoplus use whatever the fulfillment resolves to — so a gift message set once on the order shows up on the shipping label without having to be re-entered on every fulfillment split from it.