Modified setBorder() in CellWriter.php

Modified `setBorder()` so that 'top', 'left', etc. are moved up one position in the array that is passed to `setStyle()`.

`setStyle()` just directly passes the $styles to `PHPExcel_Style_Borders::applyFromArray()` which looks for `$styles['left']`, not `$styles['borders']['left']`.

Calling something like 

    $excel->row(1, function($row) {
        $row->setFontSize(14)
            ->setFontWeight('bold')
            ->setBorder('none', 'none', 'thin', 'none');
    });

previously would not apply any border because it would pass `$styles['borders']['top'] = 'none'` instead of `$styles['top'] = 'none'`.