Consider evaluating association in serializer context

For discussion:

Consider evaluating association in serializer context

That way, associations are really just anything that
can be conditionally included.  They no longer
have to actually be methods on the object or serializer.

e.g.

```diff
has_many :comments do
- last(1)
+ Comment.active.for_serialization(object).last(1)
end
```

lib/active_model/serializer/reflection.rb

@@ -8,14 +8,14 @@ module ActiveModel
     #     has_one :author, serializer: AuthorSerializer
     #     has_many :comments
     #     has_many :comments, key: :last_comments do
-    #       last(1)
+    #       object.comments.last(1)
     #     end
     #  end
     #
-    #  Notice that the association block is evaluated in the context of the association.
+    #  Notice that the association block is evaluated in the context of the serializer.
     #  Specifically, the association 'comments' is evaluated two different ways:
     #  1) as 'comments' and named 'comments'.
-    #  2) as 'comments.last(1)' and named 'last_comments'.
+    #  2) as 'object.comments.last(1)' and named 'last_comments'.
     #
     #  PostSerializer._reflections #=>
     #    # [
@@ -29,7 +29,7 @@ module ActiveModel
       # @api private
       def value(instance)
         if block
-          instance.read_attribute_for_serialization(name).instance_eval(&block)
+          instance.instance_eval(&block)
         else
           instance.read_attribute_for_serialization(name)
         end