Fix bug reassigning geocodings

[amiedes]

Fixes https://rollbar.com/carto/CartoDB/items/43590/?utm_campaign=new_item_message&utm_medium=slack&utm_source=rollbar-notification

Fixes an error trying to assign a ActiveRecord user to a Sequel geocoding.

[amiedes]

This cop is "fixed" in another PR I have pending: https://github.com/CartoDB/cartodb/pull/15920/files#diff-4f894049af3375c2bd4e608f546f8d4a0eed95464efcdea850993200db9fef5c

[amiedes]

Acceptance in staging ✅

1. Create organization with users
2. As organization user, upload dataset with geocodable information
3. Check geocodings data from the console:

irb(main):006:0> admin.username
# => "amiedes-geocodings-admin"
irb(main):007:0> user.username
# => "amiedes-geocodings-1"
irb(main):008:0> admin.geocodings.count
# => 0
irb(main):009:0> user.geocodings.count
# => 1

4. Delete organization user from the dashboard admin UI
5. Check from the console if geocodings were assigned to the admin:

irb(main):010:0> user.geocodings.count
# => 0
irb(main):011:0> admin.geocodings.count
# => 1

[amiedes]

Frontend tests failures should be unrelated 🤷‍♂️