Putting integer fields in config.case_insensitive_keys unsets them during update/validation

This took me 1 hour to debug:

```
[2] pry(main)> u = User.first
[5] pry(main)> u.valid?
=> false
[6] pry(main)> u.changes
=> {"site_id"=>[12, nil]}
[7] pry(main)>
```

I was able to trace it back to here:

```
config.case_insensitive_keys = %i[email, site_id]
```

