Fix deprecation warning in tests

Using the string version of the class reference is now deprecated when
referencing middleware. This should be written as a class not as a string.

Deprecation warning that this change fixes:
```
DEPRECATION WARNING: Passing strings or symbols to the middleware
builder is deprecated, please change
them to actual class references.  For example:
  "ActionDispatch::ShowExceptions" => ActionDispatch::ShowExceptions
```

1 files changed, 1 insertions, 1 deletions

diff --git a/railties/lib/rails/application/bootstrap.rb b/railties/lib/rails/application/bootstrap.rb
index 0f4d932749..9baf8aa742 100644
--- a/railties/lib/rails/application/bootstrap.rb
+++ b/railties/lib/rails/application/bootstrap.rb
@@ -63,7 +63,7 @@ INFO
           Rails.cache = ActiveSupport::Cache.lookup_store(config.cache_store)
 
           if Rails.cache.respond_to?(:middleware)
-            config.middleware.insert_before("Rack::Runtime", Rails.cache.middleware)
+            config.middleware.insert_before(::Rack::Runtime, Rails.cache.middleware)
           end
         end
       end