cumsum sums the groupby column
It shouldn't sum the groupby'd col (in fact index col should be the index, if groupby as_index).

```
In [13]: df = pd.DataFrame([[1, 2, np.nan], [1, np.nan, 9], [3, 4, 9]], columns=['A', 'B', 'C'])

In [14]: g = df.groupby('A')

In [16]: g.cumsum()
Out[16]: 
   A   B   C
0  1   2 NaN
1  2 NaN   9
2  3   4   9

[3 rows x 3 columns]
```

Nature of it being dispatch. Should fix up for 0.14 possibly along with some other whitelisted groupby functions.
