Change isBN type return to type guard
Change isBN type return to type guard

magny/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,borisyankov/DefinitelyTyped,magny/DefinitelyTyped