is_date is wrong for readonly cells
*Created originally on Bitbucket by [scott_stanton_veeva (Scott Stanton)](https://bitbucket.org/%7B7a9586cf-c859-4dd6-94a3-64b2af8141ac%7D/)* The implementations for Cell.is_date and ReadOnlyCell.is_date are different. ReadOnlyCell does not handle self.data_type=='d'.
issue