Updating null field value to couchbase and also getting/updating cas value using spark connector

I am using spark couchbase connector. Scala version 2.11 and couchbase connector version 2.3.0
Right now documents have one field whose value is null in dataframe. while saving into couchbase, how can i save as null. Right now field is missing in document since the field value is null.
Structure of the dataframe is like {“id”:1, “name”: “some name”, “date”: null} and in db it gets saved as {“id”:1, “name”:“some name”}. whereas i need it to be like {“id”:1, “name”: “some name”, “date”: null}

My second question is while saving to couchbase how can i get the cas and pass the cas value while writing back to db using spark dataframe. that will help me in spark not overwriting those document which is outdated