Fix isoformat(T) bug

[drdee]

This issue is caused by the line https://github.com/datadotworld/tap-redshift/blob/master/tap_redshift/__init__.py#L280
isoformat('T') only works on datetime objects not date objects.

import datetime
today = datetime.date.today()
today.isoformat()
'2019-04-26'
today.isoformat('T')
Traceback (most recent call last):
File "", line 1, in
TypeError: isoformat() takes no arguments (1 given)
now = datetime.datetime.now()
now.isoformat('T')
'2019-04-26T11:56:49.223151'
now.isoformat()
'2019-04-26T11:56:49.223151'