add in _.invoke(collection, null, [*arguments]) support where null flags for functions in a collection.

I want to support passing null to _.invoke to allow this:

test('invoke each object w/ falsy method', function(){
    var list = [function(a){return a;}, function(a){return a*2}];
    var result = _.invoke(list, null, 42);
    equal(result.join(', '), '42, 84', 'invoked on functions themselves');
});

I've added the text to the docs and made a test.