消除UITableView Cell头部间隔

1. 除去Cell 的空白间隔

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
   if ([cell respondsToSelector:@selector(setSeparatorInset:)]) {
       [cell setSeparatorInset:UIEdgeInsetsZero];
   }
   if ([cell respondsToSelector:@selector(setLayoutMargins:)]) {
       [cell setLayoutMargins:UIEdgeInsetsZero];
   }
}