Don't access @exclusive_methods if undefined (fixes #191)
Don't access @exclusive_methods if undefined (fixes #191)

kenichi/celluloid,marshall-lee/celluloid,jasonm23/celluloid,TiagoCardoso1983/celluloid,seuros/celluloid,mehta/celluloid,davydovanton/celluloid,sideci-sample/sideci-sample-celluloid,olleolleolle/celluloid,jstoja/celluloid,celluloid/celluloid,dilumn/celluloid