Make Str.subst-mutate always return a List on :g
Make Str.subst-mutate always return a List on :g

Partial fix for GH #1523, tests needed

MasterDuke17/rakudo,salortiz/rakudo,softmoth/rakudo,nbrown/rakudo,tony-o/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,lucasbuchala/rakudo,rakudo/rakudo,Gnouc/rakudo,rakudo/rakudo,Gnouc/rakudo,tbrowder/rakudo,tbrowder/rakudo,labster/rakudo,tony-o/rakudo,nbrown/rakudo,softmoth/rakudo,tony-o/rakudo,tbrowder/rakudo,labster/rakudo,lucasbuchala/rakudo,softmoth/rakudo,tbrowder/rakudo,tbrowder/rakudo,Gnouc/rakudo,rakudo/rakudo,labster/rakudo,nbrown/rakudo,Gnouc/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,tony-o/rakudo,nbrown/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,labster/rakudo,labster/rakudo,salortiz/rakudo,nbrown/rakudo,tony-o/rakudo,labster/rakudo,salortiz/rakudo,softmoth/rakudo,salortiz/rakudo,rakudo/rakudo,MasterDuke17/rakudo,nbrown/rakudo,tony-o/rakudo,MasterDuke17/rakudo,rakudo/rakudo,Gnouc/rakudo,salortiz/rakudo,lucasbuchala/rakudo,lucasbuchala/rakudo,softmoth/rakudo