Sparse data frame doesn't groupby.mean() correctly
``` python
>>>import pandas as pd
>>> pd.__version__
 '0.12.0-660-gec77315'

>>> import numpy as np
>>> np.version.version
'1.7.1'

>>> df = pd.DataFrame({'a': [0, 1, 0, 0], 'b': [0, 1, 0, 0]})
>>> sdf = df.to_sparse(fill_value=0)
>>> df.groupby('a').mean() 
    b
a   
0  0
1  1

>>> sdf.groupby('a').mean() 
    b
a   
1  0
```

I'm not surpised that the mean for group `a == 0` was not returned.  It is surprising that the result for group `a == 1` was incorrect.
