TST: Holiday tests/docs
```
>>> from pandas.tseries.holiday import next_monday_or_tuesday
>>> from pandas.tseries.holiday import AbstractHolidayCalendar
>>>from pandas.tseries.holiday import Holiday
>>>class WrongBoxingDay(AbstractHolidayCalendar):
           rules = [
               Holiday('xmas', month=12, day=25, observance=next_monday_or_tuesday),
               Holiday('boxing day', month=12, day=26, observance=next_monday_or_tuesday),
               ]
>>>import pandas as pd
>>>cal = WrongBoxingDay()
>>>cal.holidays(pd.datetime(2016,12,1), pd.datetime(2016,12,31))
DatetimeIndex(['2016-12-27', '2016-12-27'], dtype='datetime64[ns]', freq=None, tz=None)
```

The next_monday_or_tuesday function in holiday module isn't doing what it is supposed to do.
In the example above, xmas should move to Dec 27 while boxing day should stay on Dec 26, but it was also moved to Dec 27.

This bug still exits in the latest version.

How my commit works is explained in the docstring of the function.
