BUG: Fix Xl(sx|wt)Writer always using `date_format`.

Since `datetime.datetime` derives from `datetime.date` the second
`if`-clause will always trigger, leading to the `num_format_str` being
overwritten by the `date_format`.
Merge pull request #7075 from filmor/patch-1

Fix Xl(sx|wt)Writer always using date_format even if a datetime is supplied.