Clear `MRB_STR_SHARED` flag in `mrb_str_modify_keep_ascii`; close
@@ -820,6 +820,7 @@ mrb_str_modify_keep_ascii(mrb_state *mrb, struct RString *s)
if (shared->refcnt == 1 && s->as.heap.ptr == shared->ptr) {
s->as.heap.aux.capa = shared->capa;
s->as.heap.ptr[s->as.heap.len] = '\0';
+ RSTR_UNSET_SHARED_FLAG(s);
mrb_free(mrb, shared);
}
else {
