String#replace should update s->flags for MRB_STR_NO_UTF.
Otherwise String#size may return wrong length; fix
MODIFY
src/string.c
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)) {

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

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