This was failing not only for P_S, but for any engine that had
HA_PRIMARY_KEY_REQUIRED_FOR_DELETE flag set (in the tree - only P_S and federated).
Because of this flag, read_set and write_set were (possibly) changed
on update. But later the code modified these bitmaps and restored them to the default
state, losing HA_PRIMARY_KEY_REQUIRED_FOR_DELETE related changes.
sql/handler.cc:
small optimization.
don't change the *write* set only because all columns has to be *read*
