防止tableView的header悬浮

在scrollView的代理方法中实现如下代码：

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
        CGFloat sectionHeaderHeight = 30;
        if (scrollView.contentOffset.y <= sectionHeaderHeight && scrollView.contentOffset.y >= 0) {
            scrollView.contentInset = UIEdgeInsetsMake(-scrollView.contentOffset.y, 0, 0, 0);
        }else if (scrollView.contentOffset.y >= sectionHeaderHeight) {
            scrollView.contentInset = UIEdgeInsetsMake(-sectionHeaderHeight, 0, 0, 0);
        }
    }
}