Fix storing deleter for const unique ptr.

Unit test from the previous commit passes.

Author: iwanders

include/pybind11/detail/smart_holder_type_casters.h

@@ -1112,7 +1112,8 @@ struct smart_holder_type_caster<std::unique_ptr<T const, D>>
     static handle
     cast(std::unique_ptr<T const, D> &&src, return_value_policy policy, handle parent) {
         return smart_holder_type_caster<std::unique_ptr<T, D>>::cast(
-            std::unique_ptr<T, D>(const_cast<T *>(src.release())), // Const2Mutbl
+            std::unique_ptr<T, D>(const_cast<T *>(src.release()),
+                                  std::move(src.get_deleter())),  // Const2Mutbl
             policy,
             parent);
     }