bug fix for 7987

This should fix issue 7987 where there is no offset or observance.

https://github.com/pydata/pandas/issues/7987
well the doc-string doesn't say it allows None ,that's the point (it should). You can put a doc-string example of creating various holidays, this is a random example: https://github.com/pydata/pandas/blob/master/pandas/tseries/tools.py#L209