one can omit the second parameter in off

In the docs: http://goldfirestudios.com/blog/104/howler.js-Modern-Web-Audio-Javascript-Library
it says:
off: Remove custom events that you've set.
    event: String Name of event.
    function: Function (optional) The listener to remove.
Well, specifying only the event name ['load', 'end' ...] would trigger an error because it tries to fn.toString() anyway.
This commit solves that so you now can do:
sound.on('end', function(){
  alert('sound ended');
});
and later:
sound.off('end');
and it removes ALL handlers you would have for that event.