zombies wont climb stairs

in 15w50a and earlier zombies will chase you up stairs, in 15w51b they will dance at the bottom of the stairs and maybe wonder away
edit: made a test case with stairs in each direction: the zombies would climb east and west facing stairs, but not north or south facing stairs
edit: made test case with no stairs, and zombies would climb up block paths east and west, but not north or south