sync with upstream

lib/active_model/serializer/association/has_one.rb

@@ -0,0 +1,22 @@
+module ActiveModel
+  class Serializer
+    class Association
+      class HasOne < Association
+        def initialize(name, *args)
+          super
+          @root_key = @embedded_key.to_s.pluralize
+          @key ||= "#{name}_id"
+        end
+
+        def serializer_class(object, options = {})
+          serializer_from_options || serializer_from_object(object, options) || default_serializer
+        end
+
+        def build_serializer(object, options = {})
+          options[:_wrap_in_array] = embed_in_root?
+          super
+        end
+      end
+    end
+  end
+end