Bug in groupby when as_index=False
Recreation:

```
In [2]: paste
In [357]: df = DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
   .....:                        'foo', 'bar', 'foo', 'foo'],
   .....:                 'B' : ['one', 'one', 'two', 'three',
   .....:                        'two', 'two', 'one', 'three'],
   .....:                 'C' : randn(8), 'D' : randn(8)})
## -- End pasted text --

In [3]: grouped = df.groupby(['A', 'B'], as_index=False)

In [4]: x = grouped['C'].agg({ 'Q' : np.sum})
```
