add send day to private message detail api

app/views/users/private_message_details/show.json.jbuilder

@@ -7,6 +7,7 @@ json.messages do
     json.extract! message, :id, :user_id, :receiver_id, :sender_id, :content
 
     json.send_time message.display_send_time
+    json.send_day message.send_time.strftime('%Y-%m-%d')
     json.sender do
       json.partial! 'users/user_simple', user: message.sender
     end