enable memcached session store support for domain: :all config credit go...

```ruby
# All credit to https://github.com/rails/rails/pull/6084

# config/application.rb

# bundler and require stuff
module MyApp
  class Application < Rails::Application
     # config stuff

     # enable magic domain: :all support
     config.session_store :mem_cache_store, domain: :all
  end
end
```

Could core team help to put in a test case for this.