Make String#replace to check equality before modifying flags.

ref #3588

src/string.c

@@ -520,9 +520,9 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
   long len;
 
   check_frozen(mrb, s1);
+  if (s1 == s2) return mrb_obj_value(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)) {
     str_decref(mrb, s1->as.heap.aux.shared);