Display column sort indicator set through datagrid construction

This fixes the issue where setting `dataOptions.sortProperty` at
datagrid construction time did not alter the column headers to show
the sorting (though it did pass it to the `data` callback, causing
the data itself to be sorted).
Merge pull request #289 from dougwilson/fix/datagrid-initial-column-sort

Display column sort indicator set through datagrid construction