WIP: preserve Issue #479 changes

backend/app/controllers/application_controller.rb

@@ -95,7 +95,7 @@ class ApplicationController < ActionController::API
     render_validation_error base: ['すでに存在してゐます．']
   end
 
-  def render_validation_error record = nil, fields: { }, base: [], status: :unprocessable_entity
+  def render_validation_error record = nil, fields: { }, base: [], status: :unprocessable_content
     errors = { }
 
     if record