Confusing formatting after groupby
```
>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame([(3,np.datetime64('2012-07-03')),(3,np.datetime64('2012-07-04'))], columns = ['a', 'date'])
>>> print df
   a        date
0  3  2012-07-03
1  3  2012-07-04
>>> print df.groupby('a').first()
                 date
a                    
3 2012-07-03 00:00:00
```

this looks as if I had a DataFrame with indices 'a' and 3 and a column date with values '' and a datetime.
