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
```

actionpack/test/dispatch/session/mem_cache_store_test.rb

@@ -192,7 +192,7 @@ def with_test_route_set
 
         @app = self.class.build_app(set) do |middleware|
           middleware.use ActionDispatch::Session::MemCacheStore, :key => '_session_id', :namespace => "mem_cache_store_test:#{SecureRandom.hex(10)}"
-          middleware.delete "ActionDispatch::ShowExceptions"
+          middleware.delete ActionDispatch::ShowExceptions
         end
 
         yield

actionview/test/abstract_unit.rb

@@ -147,13 +147,13 @@ class ActionDispatch::IntegrationTest < ActiveSupport::TestCase
 
   def self.build_app(routes = nil)
     RoutedRackApp.new(routes || ActionDispatch::Routing::RouteSet.new) do |middleware|
-      middleware.use "ActionDispatch::ShowExceptions", ActionDispatch::PublicExceptions.new("#{FIXTURE_LOAD_PATH}/public")
-      middleware.use "ActionDispatch::DebugExceptions"
-      middleware.use "ActionDispatch::Callbacks"
-      middleware.use "ActionDispatch::ParamsParser"
-      middleware.use "ActionDispatch::Cookies"
-      middleware.use "ActionDispatch::Flash"
-      middleware.use "Rack::Head"
+      middleware.use ActionDispatch::ShowExceptions, ActionDispatch::PublicExceptions.new("#{FIXTURE_LOAD_PATH}/public")
+      middleware.use ActionDispatch::DebugExceptions
+      middleware.use ActionDispatch::Callbacks
+      middleware.use ActionDispatch::ParamsParser
+      middleware.use ActionDispatch::Cookies
+      middleware.use ActionDispatch::Flash
+      middleware.use Rack::Head
       yield(middleware) if block_given?
     end
   end

railties/lib/rails/application/bootstrap.rb

@@ -63,7 +63,7 @@ module Bootstrap
           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