UIScrollView的ContentSize 小于 fram

UIScrollView的ContentSize 小于 frame 时，无回弹效果时，只需要设置

UIScrollView * scrollView = [[UIScrollView alloc] initWithFrame:self.view.bounds];

//必须同时设置这两个属性
scrollView.bounces = YES;
scrollView.alwaysBounceVertical = YES;