fix invoice tests
fix invoice tests

TrevorHinesley/stripe-ruby-mock,sublimecoder/stripe-ruby-mock,wanelo/stripe-ruby-mock,rebelidealist/stripe-ruby-mock,vinchi777/stripe-ruby-mock,kathyonu/stripe-ruby-mock,SidestepTechnologies/stripe-ruby-mock,quantopian/stripe-ruby-mock,rebelidealist/stripe-ruby-mock,didacte/stripe-ruby-mock,dexcodeinc/stripe-ruby-mock,monkseal/stripe-ruby-mock,gravitystorm/stripe-ruby-mock,hinthealth/stripe-ruby-mock,charitywater/stripe-ruby-mock