Use Rails.application global in middleware to retrieve the config

It looks like the @app in the middleware is not an instance of `Rails::Application`
so we cannot just read config from there. But we can use the Rails global
and read configuration from there

rails_event_store/lib/rails_event_store/middleware.rb

@@ -5,8 +5,8 @@ def initialize(app)
     end
 
     def call(env)
-      if @app.config.respond_to?(:event_store)
-        @app.config.event_store.with_metadata(request_metadata(env)) do
+      if config.respond_to?(:event_store)
+        config.event_store.with_metadata(request_metadata(env)) do
           @app.call(env)
         end
       else
@@ -19,9 +19,13 @@ def request_metadata(env)
     end
 
     private
+
+    def config
+      Rails.application.config
+    end
 
     def metadata_proc
-      @app.config.x.rails_event_store.request_metadata if @app.config.x.rails_event_store.request_metadata.respond_to?(:call)
+      config.x.rails_event_store.request_metadata if config.x.rails_event_store.request_metadata.respond_to?(:call)
     end
 
     def default_request_metadata

rails_event_store/spec/middleware_spec.rb

@@ -8,9 +8,10 @@
 module RailsEventStore
   RSpec.describe Middleware do
     specify 'calls app within with_metadata block when app has configured the event store instance' do
-      app.config.event_store = event_store = Client.new
       expect(app).to receive(:call).with(dummy_env)
       middleware = Middleware.new(app)
+      ::Rails.application.config.event_store = event_store = Client.new
+      ::Rails.application.config.x = ::Rails::Application::Configuration::Custom.new
       expect(event_store).to receive(:with_metadata).with(request_id: 'dummy_id', remote_ip: 'dummy_ip').and_call_original
       middleware.call(dummy_env)
     end
@@ -22,17 +23,17 @@ module RailsEventStore
     end
 
     specify 'use config.rails_event_store.request_metadata' do
-      app.config.x.rails_event_store.request_metadata = kaka_dudu
       middleware = Middleware.new(app)
+      ::Rails.application.config.x.rails_event_store.request_metadata = kaka_dudu
 
       expect(middleware.request_metadata(dummy_env)).to eq({
         kaka: 'dudu'
       })
     end
 
     specify 'use config.rails_event_store.request_metadata is not callable' do
-      app.config.x.rails_event_store.request_metadata = {}
       middleware = Middleware.new(app)
+      ::Rails.application.config.x.rails_event_store.request_metadata = {}
 
       expect(middleware.request_metadata(dummy_env)).to eq({
         request_id: 'dummy_id',
@@ -42,6 +43,7 @@ module RailsEventStore
 
     specify 'use config.rails_event_store.request_metadata is not set' do
       middleware = Middleware.new(app)
+      ::Rails.application.config.x = ::Rails::Application::Configuration::Custom.new
 
       expect(middleware.request_metadata(dummy_env)).to eq({
         request_id: 'dummy_id',