modify any key-value pair where value is a datetime object to a string.

def _convert_datetime_str(response):
    '''
    modify any key-value pair where value is a datetime object to a string.
    '''
    if response:
        return dict([(k, '{0}'.format(v)) if isinstance(v, datetime.date) else (k, v) for k, v in six.iteritems(response)])
    return None