nullable(): add test to ensure isNullable argument is optional
nullable(): add test to ensure isNullable argument is optional
georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,borisyankov/DefinitelyTyped