Merge pull request #9331 from robertomiranda/upgrading-guide

Improve upgrading guide with ActionDispatch::BestStandardsSupport remova...

guides/source/upgrading_ruby_on_rails.md

@@ -83,6 +83,16 @@ get 'こんにちは', controller: 'welcome', action: 'index'
 ```
 
 * Rails 4.0 has removed ActionDispatch::BestStandardsSupport middleware, !DOCTYPE html already triggers standards mode per http://msdn.microsoft.com/en-us/library/jj676915(v=vs.85).aspx and ChromeFrame header has been moved to `config.action_dispatch.default_headers`
+
+Remember you must also remove any references to the middleware from your application code, for example:
+
+```ruby
+# Raise exception
+config.middleware.insert_before(Rack::Lock, ActionDispatch::BestStandardsSupport)
+```
+
+Also check your environment settings for `config.action_dispatch.best_standards_support` and remove it if present.
+
 * In Rails 4.0, precompiling assets no longer automatically copies non-JS/CSS assets from `vendor/assets` and `lib/assets`. Rails application and engine developers should put these assets in `app/assets` or configure `config.assets.precompile`.
 
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