Update code/forms/GridFieldSortableRows.php

fixes the bug mentiond in issue 24
Merge pull request #25 from flxbot/patch-1

Fixes serious issue in fixSortColumn caused in recent updates
BUGFIX: Use config layer to check for decorated $db fields
Merge pull request #26 from unclecheese/patch-1

BUGFIX: Use config layer to check for decorated $db fields