This issue was solved with changeset r42449.
Nobuyoshi, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
object.c: Module#singleton_class?
object.c (rb_mod_singleton_p): new method Module#singleton_class? to
return whether the receiver is a singleton class or not. [ruby-core:51087] [Feature #7609]