previousSelectedItemsRef.current is not updated

https://github.com/downshift-js/downshift/blob/90d6d7208c61132c01868473c6dfac3a05e92076/src/hooks/useMultipleSelection/index.js#L81

in some special cases for multiple selection when we use `setSelectedItems` instead of addSelectedItem, it can happen that previousSelectedItemsRef.current is not updated because `setSelectedItems` sets the same amount of items but different.

#### Example
1, 2, 3, 4
set 2, 3
set 1, 4
but in previousSelectedItemsRef.current are still 2, 3
set 1, 2, 3
in previousSelectedItemsRef.current are 1,2,3 because length changed