Alias create_shipment! rather than overriding it.

I never liked the fact that we were overriding spree core's
create_shipment! method. What I do here is instead to apply a hook
*after* create_shipment! has already been called to create an electronic
shipment if none exists on the order, and assign electronically
delivered inventory units to that shipment.

With a minor tweak, the existing tests all pass.