the receiver should be included in ancestors even when it's a singleton class

src/class.c

@@ -796,6 +796,8 @@ mrb_mod_ancestors(mrb_state *mrb, mrb_value self)
   struct RClass *c = mrb_class_ptr(self);
 
   result = mrb_ary_new(mrb);
+  mrb_ary_push(mrb, result, mrb_obj_value(c));
+  c = c->super;
   while (c) {
     if (c->tt == MRB_TT_ICLASS) {
       mrb_ary_push(mrb, result, mrb_obj_value(c->c));

test/t/module.rb

@@ -18,8 +18,7 @@ assert('Module#ancestors', '15.2.2.4.9') do
   end
   sc = Test4ModuleAncestors.singleton_class
   r = String.ancestors
-  r.class == Array and r.include?(String) and r.include?(Object) and
-  ! sc.ancestors.include?(sc)
+  r.class == Array and r.include?(String) and r.include?(Object)
 end
 
 assert('Module#append_features', '15.2.2.4.10') do