allow to intercept line items and request details
allow to intercept line items and request details

allows to change the paypal request by overriding the methods in a controller decorator. e.g. to allow digital items.

Fixes #86

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