Introduce a new method for checking whether session_store is set by user or by Rails

- We need to ability to check whether the session store it is set or
   not so that we can set it ourselves in an internal initializer.
- We can't rely on return value of `config.session_store` as it can
  return `nil` when set to `disabled` and we will accidentally override it
  and set to default cookie store.
- So introduced new method which just tells us whether it is set or not.

railties/lib/rails/application/configuration.rb

@@ -188,6 +188,10 @@ def session_store(*args)
         end
       end
 
+      def session_store? #:nodoc:
+        @session_store
+      end
+
       def annotations
         SourceAnnotationExtractor::Annotation
       end