* gc.c (internal_object_p): should not expose singleton classes
  without a metaclass.  based on patches by ko1 and shugo.
  [Bug #11740]
* class.c (rb_singleton_class_object_p): added.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53243 b2dd03c8-39d4-4d8f-98ff-823fe69b080e