BUG: Rolling apply on DataFrame with Datetime index returns NaN
 - [x]  #15305
 - [x] changing the order of 'calling roll_sum' and 'get_window_indexer' in 'roll_generic' could fix #15305 
The bug caused by the return values from get_window_indexer. It changes value of 'win', which is not supposed to be passed to roll_sum.


