Bug in rolling_mean when minp == win
``` python
In [11]: mom.rolling_mean(s,4,min_periods=3)
Out[11]: 
0   NaN
1   NaN
2     1
3     1
4     1
5     1
6     1
7     1
8     1
9     1
dtype: float64
#but
In [12]: mom.rolling_mean(s,4,min_periods=4)
Out[12]: 
0   NaN
1   NaN
2   NaN
3     1
4     1
5   NaN
6   NaN
7   NaN
8   NaN
9     1
dtype: float64
```

one more discovered during #2953
