Should EmitterMixin check for undefined callbacks

[jodator]

Currently such test will fail:

it( 'should not fail on empty callback', () => {
	listener.listenTo( emitter, 'foo' );

	emitter.fire( 'foo' );
	emitter.fire( 'foo:bar' );
} );

As passing undefined here is wrong anyway should we check callback in listenTo() and simply ignoring it?

[Reinmar]

It could :) The same for a couple thousands other methods :D