BUG: DateOffset weekday around DST produces unexpected results. fixes #5175

closes #5175

Wound up working on this in preparation for the DST change next week. Here's a first pass at a solution and a test.
I couldn't find any tests or documentation for the singular form anywhere in pandas. However, I think this behavior is inherited from ``dateutil.relativedelta``, which [mentions it in passing in the documentation](https://labix.org/python-dateutil#head-ba5ffd4df8111d1b83fc194b97ebecf837add454): 

    year, month, day, hour, minute, second, microsecond
        Absolute information.
    years, months, weeks, days, hours, minutes, seconds, microseconds
        Relative information, may be negative.