BUG? DataFrame.groupby.apply result has unexpected index
Compare the output of these three groupby/apply calls:

```
In [66]: df = pd.DataFrame({'a': [1, 1, 2, 2], 'b': range(4)})

In [67]: df
Out[67]:
   a  b
0  1  0
1  1  1
2  2  2
3  2  3

In [68]: df.groupby('a')['b'].apply(lambda x: 2 * x)
Out[68]:
0    0
1    2
2    4
3    6
dtype: int64

In [69]: df.groupby('a').apply(lambda x: 2 * x)['b']
Out[69]:
0    0
1    2
2    4
3    6
Name: b, dtype: int64

In [70]: df.groupby('a').apply(lambda x: 2 * x['b'])
Out[70]:
a
1  0    0
   1    2
2  2    4
   3    6
Name: b, dtype: int64
```

The first two are what I expected; the last is not -- I expected the same result as the previous two.
