singleton class needs to set __outer__; fixed #1983

src/class.c

@@ -982,6 +982,10 @@ mrb_singleton_class(mrb_state *mrb, mrb_value v)
   }
   obj = mrb_basic_ptr(v);
   prepare_singleton_class(mrb, obj);
+  if (mrb->c && mrb->c->ci && mrb->c->ci->target_class) {
+    mrb_obj_iv_set(mrb, (struct RObject*)obj->c, mrb_intern_lit(mrb, "__outer__"),
+                   mrb_obj_value(mrb->c->ci->target_class));
+  }
   return mrb_obj_value(obj->c);
 }