BUG: groupby.agg with mutliples ignores as_index=False after subset to a single column

```
df = pd.DataFrame({'a': [1, 1, 2], 'b': [3, 4, 5]})
gb = df.groupby('a', as_index=False)['b']
result = gb.agg(["sum", "mean"])
print(result)
#    sum  mean
# a           
# 1    7   3.5
# 2    5   5.0
```

In the output, `a` should be a column rather than in the index.