partialUpdateObject remove some fields

- Algolia Client Version: 3.16.5
- Language Version: JAVA

### Description
We implemented a new agent which update existing items from an Algolia index and we observed that partialUpdateObject does not work as expected : some fields, not declared in attributesToRetrieved, are removed when saving an updated item.

### Steps To Reproduce
1. Having an Algolia index with data (ex: [objectID, title, text, url, source, date, subSource])
2. Set attributesToRetrieved with back office configuration (ex: [objectID, title, text, url, source])
3. Retrieve items from this index with more restrictions on attributesToRetrieved (ex: [objectID, title, text])
4. Add a new field to item (ex: score)
5. Save item with partialUpdateObject with only changed fields (ex:[objectID, title, score])
=> Items saved only have [objectID,title,text, url, source, score] fields and no more [date, subSource] fields

Comienza reproduciendo el caso reportado con partialUpdateObject del cliente Java y attributesToRetrieve restringido. Inspecciona el punto de entrada de la actualización parcial y sus pruebas existentes, si las hay, para determinar por qué se eliminan los campos omitidos del objeto recuperado; se considera completado cuando se actualizan los campos seleccionados y se conservan date y subSource.

Escrito por el modelo de indexación a partir del texto del issue.