incorrect calculation of centered moving averages for even length series
For even numbered periods the centered moving average is calculated incorrectly. Suppose the period length is 5. Then the center of 5 periods is 3. However if the period length is 4 then the center of the period is 2.5. The value at index 3 should be the average of the values at 2.5 and 3.5. Pandas is showing the 2.5 value at 3 which is incorrect.

EXAMPLE:
a=pd.Series(range(1,6), index=range(1,6))
a.rolling(4, center=True).mean()

1    NaN
2    NaN
3    2.5
4    3.5
5    NaN
