Update item into hash table with specified key and item.
If key is already present, destroys old item and inserts new one.
Use free_fn method to ensure deallocator is properly called on item.

def update(self, key, item):
        """
        Update item into hash table with specified key and item.
If key is already present, destroys old item and inserts new one.
Use free_fn method to ensure deallocator is properly called on item.
        """
        return lib.zhash_update(self._as_parameter_, key, item)