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`.
