TableView plain方式下header不悬浮

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    if (scrollView == self.tableView)
    {
        CGFloat sectionHeaderHeight = 20;
        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);
        }
    }
}