I have added the specs for rb_undef_method.
Hmm... But this refactoring is difficult for me.
I couldn't find smart way.

undef_method

1. no arguments and no exceptions
2. check coercion
3. check frozen
4. check method defined
5. undef

undef_method!(rb_undef_method)

1. check frozen 
2. undef

instance_method

1. check coercion
2. check method defined