Updates to an upserted record are not applied

After upserting, the very next save on the resulting record does not persist

```ruby
MyModel.find_by(uuid: 1234) # nil

record = MyModel.new # (id, uuid, name, age)
record.uuid = 1234 # uuid is the upsert_key
result = record.upsert # creates record with uuid 1234

result.update(name: "foo")

persisted = MyModel.find_by(uuid: 1234)
persisted.name # nil

result.update(age: 42)

persisted = MyModel.find_by(uuid: 1234)
persisted.age # 42
```

The first upsert does an `INSERT ON CONFLICT` generating the record
The very next update tries to update every column `UPDATE "my_model" SET "id" = $1, "name" = $2 ...`
Then the update after that does the usual `UPDATE "my_model" SET "age" = $1 ...`

Start with the upsert flow that creates the record, then inspect how the resulting record tracks persisted attributes before the first update. Reproduce the example in the issue and verify that the first result.update(name: "foo") persists name, while the later age update continues to persist normally.

Written by the indexing model from the issue text.