added explicit test to check whether values are no longer observed even if subscribed multiple times
added explicit test to check whether values are no longer observed even if subscribed multiple times

dschalk/mobservable,mobxjs/mobx,andykog/mobservable,dschalk/mobservable,andykog/mobservable,mweststrate/MOBservable,dschalk/mobservable,mweststrate/MOBservable,romseguy/mobservable,mobxjs/mobx,sandals/mobservable,ryanbraganza/MOBservable,mobxjs/mobx,andykog/mobx,sandals/mobservable,andykog/mobx,romseguy/mobservable,mweststrate/MOBservable,sandals/mobservable,mobxjs/mobx,ryanbraganza/MOBservable,romseguy/mobservable,ryanbraganza/MOBservable