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

In the `XlsxWriter` and `XlwtWriter` implementations there is a minor bug resulting in the `datetime_format` being discarded (i.e. https://github.com/pydata/pandas/blob/master/pandas/io/excel.py#L662):

    if isinstance(cell.val, datetime.datetime):
        num_format_str = self.datetime_format
    if isinstance(cell.val, datetime.date):
        num_format_str = self.date_format

Since `datetime.datetime` derives from `datetime.date` the second `if`-clause in the  will always trigger, leading to the `num_format_str` being overwritten by the `date_format`.