Allow serialized ID to be overwritten for belongs-to relationships

If the `id` attribute for a class isn't taken directly from the object when
serializing it, it may be desirible for other classes that serialize a
relationship with that class to overwrite the relationship IDs they serialize.

For example, suppose we have:

```(ruby)
class Repo < Model
  attributes :id, :github_id, :name
  associations :configs
end

class Config < Model
  attributes :id
  belongs_to :repo
end

class RepoSerializer < ActiveModel::Serializer
  attributes :id, :name

  has_many :update_configs

  def id
    object.github_id
  end
end

class ConfigSerializer < ActiveModel::Serializer
  attributes :id
  belongs_to :repo
end
```

In the above example, serializing a list of `Repo`s will give the `github_id`
for each one, but serializing a `Config` will give the `id` for its parent repo.

Ideally AMS would inspect the `RepoSerializer` when serializing the `Config`,
and realise it can't just output the foreign key. Unfortunately, getting the
serialization class for the child repo currently requires loading the record
(via evaluating `lazy_assocation`), and loses the performance benefit of the
existing `belongs_to?` path. Instead, I've opted to use
`read_attribute_for_serialization` instead of `object.send` to fetch the
serialized foreign key. This allows the serialized relationship ID to be
overwritten using

```(ruby)
class ConfigSerializer < ActiveModel::Serializer
  ...

  def repo_id
    object.repo.github_id
  end
end
```

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -45,7 +45,7 @@ module ActiveModelSerializers
         def data_for_one(association)
           if association.belongs_to? &&
               parent_serializer.object.respond_to?(association.reflection.foreign_key)
-            id = parent_serializer.object.send(association.reflection.foreign_key)
+            id = parent_serializer.read_attribute_for_serialization(association.reflection.foreign_key)
             type = association.reflection.type.to_s
             ResourceIdentifier.for_type_with_id(type, id, serializable_resource_options)
           else