Use association value for determining serializer used

Ensures overridden association value works when orignal association does not return a result.

test/adapter/json/belongs_to_test.rb

@@ -32,7 +32,7 @@ module ActiveModel
             serializer = PostSerializer.new(@anonymous_post)
             adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
 
-            assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], blog: nil, author: nil}, adapter.serializable_hash)
+            assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], blog: {id: 999, name: "Custom blog"}, author: nil}, adapter.serializable_hash)
           end
         end
       end

test/adapter/json/collection_test.rb

@@ -45,7 +45,10 @@ module ActiveModel
                 id: 1,
                 name: "Steve K."
               },
-              blog: nil
+              blog: {
+                id: 999,
+                name: "Custom blog"
+              }
             }]
             assert_equal expected, @adapter.serializable_hash
           end