HasOne nil object should return [] under embedded key.

Ensure that @wrap_in_array is always respected when set to true even
when associated object is nil.

lib/active_model/default_serializer.rb

@@ -9,12 +9,15 @@ module ActiveModel
 
     attr_reader :object
 
-    def initialize(object, options=nil)
+    def initialize(object, options={})
       @object = object
+      @wrap_in_array = options[:_wrap_in_array]
     end
 
     def as_json(options={})
-      @object.as_json
+      return [] if @object.nil? && @wrap_in_array
+      hash = @object.as_json
+      @wrap_in_array ? [hash] : hash
     end
     alias serializable_hash as_json
     alias serializable_object as_json

lib/active_model/serializer.rb

@@ -183,7 +183,7 @@ end
     end
 
     def serializable_object(options={})
-      return nil if object.nil?
+      return @wrap_in_array ? [] : nil if @object.nil?
       hash = attributes
       hash.merge! associations
       @wrap_in_array ? [hash] : hash

test/unit/active_model/serializer/has_one_test.rb

@@ -106,6 +106,21 @@ module ActiveModel
         }, @user_serializer.serializable_hash)
       end
 
+      def test_associations_embedding_nil_ids_including_objects_serialization_using_as_json
+        @association.embed = :ids
+        @association.embed_in_root = true
+        @user.instance_eval do
+          def profile
+            nil
+          end
+        end
+
+        assert_equal({
+          'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => nil },
+          'profiles' => []
+        }, @user_serializer.as_json)
+      end
+
       def test_associations_embedding_ids_including_objects_serialization_using_as_json
         @association.embed = :ids
         @association.embed_in_root = true