partial slicing FAILS with a datetimeindex 
Hi,

I have a simple dataframe with a multiindex. 

```
stats.index.names
Out[67]: FrozenList([u'day', u'category'])
```

day is a time variable, category is a string.

```
stats.index.inferred_type
Out[69]: 'mixed'

stats.index.dtype_str
Out[68]: 'object'
```

if I type

```
idx=pd.IndexSlice
stats.loc[idx['2015-01-01',:],:]
```

I get the correct slice at '2015-01-01', that is all the observations for all the categories on that day.

If I type

```
idx=pd.IndexSlice
stats.loc[idx['2015',:],:] 
```

I get back (almost) all my observations, even for years other than 2015.

What can be the problem here? I cannot paste the data unfortunately but I am happy to help in any way.

Thanks
