String#replace should update s->flags for MRB_STR_NO_UTF.

Otherwise String#size may return wrong length; fix #3448

src/string.c

@@ -519,6 +519,8 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
   long len;
 
   check_frozen(mrb, s1);
+  s1->flags &= ~MRB_STR_NO_UTF;
+  s1->flags |= s2->flags&MRB_STR_NO_UTF;
   if (s1 == s2) return mrb_obj_value(s1);
   len = RSTR_LEN(s2);
   if (RSTR_SHARED_P(s1)) {