Method invoice_as_pdf needs to be added back after a recent PR merge
where it was removed. Method was also re-factored to rescue from
possible exception raised in check_response method. The status code can't just be
checked as previously since check_response will raise errors on any
non 200 status code response.
use 'it' method instead for style consistency
spec refactored Online::Service::Invoice invoice_as_pdf method
Add back qb_secret from recent PR but instead properly pass it in as the second arg of OAuth::Consumer.new