Update ci->mid according to surrounding scope; fix #3490

Author: matz

src/vm.c

@@ -569,6 +569,7 @@ eval_under(mrb_state *mrb, mrb_value self, mrb_value blk, struct RClass *c)
   p = mrb_proc_ptr(blk);
   ci->proc = p;
   ci->argc = 1;
+  ci->mid = ci[-1].mid;
   if (MRB_PROC_CFUNC_P(p)) {
     stack_extend(mrb, 3, 0);
     mrb->c->stack[0] = self;