Unscope includes when plucking linkage data

It's very common to eager load data in JR. But if we keep includes
values when plucking for linkage data, it can make the query
unnecessarily expensive.

Let's use Post model for example:

If we directly pluck all posts' `created_at` column, the query would be
very simple

```sql
SELECT "posts"."created_at" FROM "posts"
```

But if we keep includes values in the previous relation object, like

```ruby
Post.includes(:author).pluck(:created_at)
```

It generates unnecessarily complicated query

```sql
SELECT "posts"."created_at" FROM "posts" LEFT OUTER JOIN "people" ON
"people"."id" = "posts"."author_id"
```

So we should always unscope includes when we only want to pluck linkage
data.

Author: st0012

lib/jsonapi/resource_serializer.rb

@@ -410,6 +410,7 @@ def to_many_linkage(source, relationship)
             [obj.type.underscore.pluralize, obj.id]
           end
         else
+          assoc = assoc.unscope(:includes) if assoc.is_a?(ActiveRecord::Relation)
           assoc.pluck(:type, :id).map do |type, id|
             [type.underscore.pluralize, id]
           end