Index.asof returns different types
depending on whether the date exists or not:

In [42]: idx.asof(datetime(2008, 5, 2))
Out[42]: datetime.datetime(2008, 5, 2, 0, 0)

In [43]: idx.asof(datetime(2008, 5, 3))
Out[43]: <Timestamp: 2008-05-02 00:00:00>
