BUG: Series.any/all with level keyword should preserve boolean dtype
```
In [5]: s = pd.Series([False, False, True, True, False, True],
                      index=[0, 0, 1, 1, 2, 2], dtype="boolean")  

In [6]: s.all(level=0) 
Out[6]: 
0    False
1     True
2    False
dtype: bool
```

The dtype of the result could also be boolean instead of bool.