String#replace should check replacing string; fix
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@@ -519,6 +519,7 @@ 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);
len = RSTR_LEN(s2);
if (RSTR_SHARED_P(s1)) {
str_decref(mrb, s1->as.heap.aux.shared);
