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.

As described in the doc, `next_monday_or_tuesday` moves both Sunday (Dec 25) and Monday (Dec 26) to Tuesday. The correct `Calendar` definition should be:

```
class WrongBoxingDay(AbstractHolidayCalendar):
    rules = [
               Holiday('xmas', month=12, day=25, observance=next_monday),
               Holiday('boxing day', month=12, day=26, observance=next_monday_or_tuesday),
               ]
cal.holidays(pd.datetime(2016,12,1), pd.datetime(2016,12,31))
# DatetimeIndex(['2016-12-26', '2016-12-27'], dtype='datetime64[ns]', freq=None)
```

Appreciated if you improve the doc.
 Thanks @sinhrks. I see your definition doesn't cause both xmas and boxing day moving to tuesday. Maybe my commit just a bit less confusing (maybe only to myself :) ) as when you see two adjacent holidays you just need to assign next_monday_or_tuesday to both holidays.
 @enfeizhan the `Holiday` class is missing lots of tests for things like this. So going to reprupose this PR for tests and/or doc updates. Behavior should remain unchanged.
 pls rebase if you'd like to rebase / update and add some tests
