QST: `min_periods` effect on `std` rolling aggregation?
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#### Question about pandas

In rolling aggregation, `min_periods` dictates that if there's `N` valid observations, and that `N >= min_periods`, the aggregation should be computed with all valid observations in the window. However, for `std` and `var` aggregation, the above guideline is not observed:

```python
# Your code here, if applicable
s = pd.Series([1.0, 1.0, 1.0, np.nan, 1.0, 2.0])
s.rolling(2, min_periods=1).std()
0         NaN
1    0.000000
2    0.000000
3         NaN
4         NaN
5    0.707107
dtype: float64
```

The question is: should `min_periods` have effects on `std` aggregation?