Do not consider optional belongs_to associations when searching for an owner

[felipefreitag]

Found on TCC:

• message belongs_to offers_diamond, optional: true
• message belongs_to offer

Resulting SQL for #index:
"SELECT "messages".* FROM "messages" INNER JOIN "offers_diamonds" ON "offers_diamonds"."id" = "messages"."offers_diamond_id" INNER JOIN "offers" ON "offers"."id" = "offers_diamonds"."offer_id" WHERE "offers"."user_id" = 48 AND "messages"."offer_id" = 41 ORDER BY "messages"."created_at" DESC"

[felipefreitag]

It will fail on index, update and delete actions.

Create a scenario where the optional association comes first on the association list. Probably ordered by migration.

https://github.com/SeasonedSoftware/croods-rails/blob/1266790a7a61c0812b7edafb5526452a535d7361/lib/croods/policy/scope.rb#L38

https://github.com/SeasonedSoftware/croods-rails/blob/1266790a7a61c0812b7edafb5526452a535d7361/lib/croods/policy.rb#L67

Reject optional associations: association.options[:optional]