Problem constructing Series from dict with datetime.date in level of MultiIndex
When calling the Series constructor on a dict with tuple keys containing a datetime.date object 
the values are converted to NaNs as this example demonstrates:

```
In [52]:

pd.Series({(1, datetime.date(2012, 1, 1)): 1, 
           (2, datetime.date(2012, 1, 2)): 2})
Out[52]:
1  2012-01-01   NaN
2  2012-01-02   NaN
```

It only happens with MultiIndex but otherwise arises regardless of the number and order of the levels. 
I know having datetime.date objects in an index is not optimal and that calling the constructor in this way actually forces the dates to Timestamp objects but in my use case I'm receiving data which is structured this way and converting it to Pandas objects. 
