Add customer.upcoming_invoice convenience method
Add customer.upcoming_invoice convenience method

It seems to embrace the 'Ruby Way' (and more convenient in my own code) to be able to access an upcoming invoice from the customer itself rather than de-reference the customer id and ask the Invoice class itself for that info.
theicfire/stripe-ruby,OutlawAndy/stripe-ruby,finbarr/kickstarter_stripe,alkesh26/stripe-ruby,ajporterfield/stripe-ruby,PivotDesk/stripe-ruby,avvo/stripe-ruby,HashNuke/stripe-ruby,molawson/stripe-ruby,Easybring/stripe-ruby,iloveitaly/stripe-ruby,hmcfletch/stripe-ruby,gizm0duck/stripe-ruby,nbrustein/stripe-ruby,thestorefront/stripe-ruby,stripe/stripe-ruby