DataFrame.index.asof() bug
An example is best:

``` python
2 Inn: df = P.DataFrame(index=P.date_range('2014-9-6 1:30', '2014-9-9', freq='6H'))
3 Inn: df
3 Out: 


2014-09-06 01:30:00
2014-09-06 07:30:00
2014-09-06 13:30:00
2014-09-06 19:30:00
2014-09-07 01:30:00
2014-09-07 07:30:00
2014-09-07 13:30:00
2014-09-07 19:30:00
2014-09-08 01:30:00
2014-09-08 07:30:00
2014-09-08 13:30:00
2014-09-08 19:30:00
2014-09-09 01:30:00

4 Inn: df.index.asof('2014-9-8')  # wrong
4 Out: Timestamp('2014-09-08 00:00:00')

5 Inn: df.index.asof(P.to_datetime('2014-9-8'))  # right
5 Out: Timestamp('2014-09-07 19:30:00', offset='6H')

7 Inn: P.version.version
7 Out: '0.14.1'
```

df.index.asof() appears to know how to handle datetime strings, but it cant
