datetime slices with multiindex unexplained behaviour
I don't think this is related to  #9732 or #10331, but maybe I'm missing something.

``` python
index = pd.MultiIndex.from_product([pd.date_range('2015-01-01', '2015-04-01', freq='8H'), ['A', 'B', 'C']], names=['level1', 'level2'])
s = pd.Series(np.random.randn(813), index=index)
```

Now I'm trying to select a slice from s as follows:

```
idx = pd.IndexSlice
s_small = s.loc[idx['2015-01-01':'2015-01-08', :]]
```

Now this is very strange, if I select a slice smaller than 8 days then it  works fine, but for anything bigger the selection doesn't work:

```
>>> s.loc[idx['2015-01-01':'2015-01-08', :]].shape #(8 days times 3 8h periods times 3 levels)
(72,)
>>> s.loc[idx['2015-01-01':'2015-01-09', :]].shape #???
(569,)
>>>
```
