Confusing - rolling min( ) function "accepting" objects
#### Code Sample, a copy-pastable example if possible

```python
s = pd.Series([3,1,2,'x','y','z'])

s.rolling(2).min()

0    3
1    1
2    2
3    x
4    y
5    z
dtype: object

```
#### Problem description

Should raise an exception because the series contains objects. Instead, function returns the series.