Make customisation of completion route simple
Make customisation of completion route simple

mirroring Spree::CheckoutController#update

Fixes #62

Boomkat/better_spree_paypal_express,StemboltHQ/solidus_paypal_express,quentinuys/better_spree_paypal_express,ZEITIO/better_spree_paypal_express,N-SARAVANAKUMAR/spree_paypal_express,N-SARAVANAKUMAR/spree_paypal_express,freerunningtech/solidus_paypal_express,madetech/better_spree_paypal_express,givanse/better_spree_paypal_express,watg/better_spree_paypal_express,watg/better_spree_paypal_express,Lostmyname/better_spree_paypal_express,StemboltHQ/solidus_paypal_express,givanse/better_spree_paypal_express,Reformation/better_spree_paypal_express,watg/better_spree_paypal_express,Lostmyname/better_spree_paypal_express,FlexMessenger/better_spree_paypal_express,quentinuys/better_spree_paypal_express,FlexMessenger/better_spree_paypal_express,Lostmyname/better_spree_paypal_express,Boomkat/better_spree_paypal_express,formrausch/better_spree_paypal_express,freerunningtech/solidus_paypal_express,Boomkat/better_spree_paypal_express,StemboltHQ/solidus_paypal_express,spree-contrib/better_spree_paypal_express,madetech/better_spree_paypal_express,givanse/better_spree_paypal_express,FlexMessenger/better_spree_paypal_express,freerunningtech/solidus_paypal_express,Reformation/better_spree_paypal_express,formrausch/better_spree_paypal_express,quentinuys/better_spree_paypal_express,spree-contrib/better_spree_paypal_express,spree-contrib/better_spree_paypal_express,ZEITIO/better_spree_paypal_express,madetech/better_spree_paypal_express