Simplify lookup_store

I think allowing nested array parameters is almost useless.

activesupport/lib/active_support/cache.rb

@@ -53,8 +53,7 @@ module ActiveSupport
       #
       #   ActiveSupport::Cache.lookup_store(MyOwnCacheStore.new)
       #   # => returns MyOwnCacheStore.new
-      def lookup_store(*store_option)
-        store, *parameters = *Array.wrap(store_option).flatten
+      def lookup_store(store = nil, *parameters)
 
         case store
         when Symbol

railties/lib/rails/application/bootstrap.rb

@@ -56,7 +56,7 @@ module Rails
       # Initialize cache early in the stack so railties can make use of it.
       initializer :initialize_cache, group: :all do
         unless Rails.cache
-          Rails.cache = ActiveSupport::Cache.lookup_store(config.cache_store)
+          Rails.cache = ActiveSupport::Cache.lookup_store(*config.cache_store)
 
           if Rails.cache.respond_to?(:middleware)
             config.middleware.insert_before(::Rack::Runtime, Rails.cache.middleware)

railties/test/application/middleware_test.rb

@@ -239,9 +239,10 @@ module ApplicationTests
     end
 
     test "Rails.cache does not respond to middleware" do
-      add_to_config "config.cache_store = :memory_store"
+      add_to_config "config.cache_store = :memory_store, { timeout: 10 }"
       boot!
       assert_equal "Rack::Runtime", middleware[5]
+      assert_instance_of ActiveSupport::Cache::MemoryStore, Rails.cache
     end
 
     test "Rails.cache does respond to middleware" do