Ruby

string.c: should not taint fstring

• string.c (rb_obj_as_string): fstring should not be infected.
  re-apply r52872 and fix a typo.
  TODO: other frozen strings also may not be.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52882 b2dd03c8-39d4-4d8f-98ff-823fe69b080e