Add Solidus install generator routing fix

A second run of the generator mounts Spree::Core::Engine a second time.

lib/alchemy/solidus/engine.rb

@@ -32,6 +32,7 @@ class Engine < ::Rails::Engine
 
         if SolidusSupport.solidus_gem_version < Gem::Version.new('2.5')
           require 'alchemy/solidus/spree_custom_user_generator_fix'
+          require 'alchemy/solidus/spree_install_generator_fix'
         end
       end
     end

lib/alchemy/solidus/spree_install_generator_fix.rb

@@ -0,0 +1,29 @@
+require 'generators/spree/install/install_generator'
+
+Spree::InstallGenerator.class_eval do
+  CORE_MOUNT_ROUTE = "mount Spree::Core::Engine"
+
+  def install_routes
+    routes_file_path = File.join('config', 'routes.rb')
+    unless File.read(routes_file_path).include? CORE_MOUNT_ROUTE
+      insert_into_file routes_file_path, after: "Rails.application.routes.draw do\n" do
+        <<-ROUTES
+  # This line mounts Solidus's routes at the root of your application.
+  # This means, any requests to URLs such as /products, will go to Spree::ProductsController.
+  # If you would like to change where this engine is mounted, simply change the :at option to something different.
+  #
+  # We ask that you don't use the :as option here, as Solidus relies on it being the default of "spree"
+  #{CORE_MOUNT_ROUTE}, at: '/'
+
+ROUTES
+      end
+    end
+
+    unless options[:quiet]
+      puts "*" * 50
+      puts "We added the following line to your application's config/routes.rb file:"
+      puts " "
+      puts "    #{CORE_MOUNT_ROUTE}, at: '/'"
+    end
+  end
+end