- Fix #54132
- ### Problem

  If an application sets its cookie serializer to message pack, it is not
  possible to add a SafeBuffer in its flash message.

  ```ruby
  class PostController < ApplicationController
    def index
      flash.notice = t(:hello_html) # Raises an error
    end
  end
  ```

  ### Solution

  Add SafeBuffer to the list of registered messagepack types.