fix(xo-server/subjects/addToArraySet): dont erase previous values (#5269)
fix(xo-server/subjects/addToArraySet): dont erase previous values (#5269)

Prior to this change, adding a value to an existing set that already contains
that value would replace the whole set with a new one containing only that
value.
lmcro/xo-web,vatesfr/xo-web,vatesfr/xo-web,lmcro/xo-web,lmcro/xo-web