fixing the find or delete serde inconsistency

[mlokr]

Hello,

I found an inconsistency in the way mongo documents are deserialized when using serde(with) and find_one_and_delete. My fix might nor be ideal because it breaks

< operation::find_and_modify::test::handle_no_value_delete                                                                                                                                                                                    
< operation::find_and_modify::test::handle_no_value_replace                      
< operation::find_and_modify::test::handle_no_value_update

The tests no longer return Err(_) but just Ok(None). I am not sure how I can assert that Field is present but can be null, when deserializing Option<T> and could not find anything either.

I added a tests to verify my changes fix the issue (see changes)

[abr-egn]

Hi!

Thanks for reporting this! I particularly appreciate the included test case. At a first guess, the issue here is similar to others we've fixed recently where the helper function you're using has different logic for human-readable vs non-human-readable formats, and the driver code inadvertently serializes with one and deserializes with the other.

I'm going to reproduce the issue on my end and, if that's the case, it should be straightforward to adapt your PR to fix this without breaking the tests you note :)

[abr-egn]

I couldn't find a way to distinguish between null and "missing" in autogenerated Serde impls either; fortunately, we can just swap the Bson for a RawBson and check directly :)