Don't treat has_one associations embed objects as an array

Closes #442

Thanks @arrtchiu for reporing and giving a test case

test/unit/active_model/serializer/has_many_test.rb

@@ -130,7 +130,7 @@ module ActiveModel
         }, @post_serializer.as_json)
       end
 
-      def test_associations_using_a_given_array_serializer
+      def test_associations_embedding_ids_using_a_given_array_serializer
         @association.embed = :ids
         @association.embed_in_root = true
         @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
@@ -144,6 +144,18 @@ module ActiveModel
           comments: { my_content: ['fake'] }
         }, @post_serializer.as_json)
       end
+
+      def test_associations_embedding_objects_using_a_given_array_serializer
+        @association.serializer_class = Class.new(ActiveModel::ArraySerializer) do
+          def serializable_object
+            { my_content: ['fake'] }
+          end
+        end
+
+        assert_equal({
+          'post' => { title: 'Title 1', body: 'Body 1', comments: { my_content: ['fake'] } }
+        }, @post_serializer.as_json)
+      end
     end
   end
 end

test/unit/active_model/serializer/has_one_test.rb

@@ -116,7 +116,7 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
-      def test_associations_using_a_given_serializer
+      def test_associations_embedding_ids_using_a_given_serializer
         @association.embed = :ids
         @association.embed_in_root = true
         @association.serializer_class = Class.new(ActiveModel::Serializer) do
@@ -132,6 +132,20 @@ module ActiveModel
           'profiles' => [{ name: 'fake' }]
         }, @user_serializer.as_json)
       end
+
+      def test_associations_embedding_objects_using_a_given_serializer
+        @association.serializer_class = Class.new(ActiveModel::Serializer) do
+          def name
+            'fake'
+          end
+
+          attributes :name
+        end
+
+        assert_equal({
+          'user' => { name: 'Name 1', email: 'mail@server.com', profile: { name: 'fake' } }
+        }, @user_serializer.as_json)
+      end
     end
   end
 end