Add specs to Virtus::AttributeSet#[]= that remove some mutations
Add specs to Virtus::AttributeSet#[]= that remove some mutations

* There are still 2 other mutations, but this resolves the ones related to the
  string/symbol indexes.

macressler/virtus,kristianmandrup/virtus,baojjeu/virtus,hapiben/virtus,solnic/virtus,nuclearsandwich/virtus,reevoo/virtus,lgierth/virtus,mallikarjunayaddala/virtus,JustMikey/virtus,artemeff/virtus,dslh/virtus,XescuGC/virtus,faucct/virtus