add missing method to product.rb, fixes #241.
add missing method to product.rb, fixes #241.

tryshoppe/shoppe,thierry-rietsch/shoppe,hilquias/shoppe,mmanesh/shoppe,tryshoppe/shoppe,amaniak/shoppe,aniltimt/shoppe,allisonjulian/shoppe,Invision70/shoppe,farukaydin/shoppe,PhdDone/shoppe,totalmeinhof/shoppe,PhdDone/shoppe,diraulo/shoppe,amaniak/shoppe,adityasapate/shoppe,totalmeinhof/shoppe,amaniak/shoppe,svenyurgensson/shoppe,PhdDone/shoppe,hilquias/shoppe,chrztoph/shoppe,aniltimt/shoppe,jsantana90/shoppe,farukaydin/shoppe,juan267/shoppe,thierry-rietsch/shoppe,likelazyeyes/shoppe,totalmeinhof/shoppe,mmanesh/shoppe,juan267/shoppe,farukaydin/shoppe,allisonjulian/shoppe,juan267/shoppe,diraulo/shoppe,adityasapate/shoppe,diraulo/shoppe,likelazyeyes/shoppe,Invision70/shoppe,Invision70/shoppe,allisonjulian/shoppe,aniltimt/shoppe,svenyurgensson/shoppe,jsantana90/shoppe,svenyurgensson/shoppe,tryshoppe/shoppe,hilquias/shoppe,chrztoph/shoppe,adityasapate/shoppe,mmanesh/shoppe,likelazyeyes/shoppe,jsantana90/shoppe,thierry-rietsch/shoppe,chrztoph/shoppe