bugfix: borders were not written correctly in word2007

Wrong index caused that for example the bottom border for a cell was not written correctly:

test with: 
```php
    $cell = $row->addCell(Font::centimeterSizeToTwips(6.9), array(
      'bgColor'=>'6BA823',
      'borderRightColor'=>'efefef',
      'borderRightSize'=>Font::pointSizeToTwips(2),
      'borderBottomColor'=>'efefef',
      'borderBottomSize'=>Font::pointSizeToTwips(2),
    ));
```
will only display the rightBorder but not the bottomBorder