Make app_name consistent between new and update

Previously, the app_name in the App Generator would behave differently
when creating a new app and updating an existing app.

When updating an app, app_name is always the Rails::Application class
name underscored. When creating a new app, app_name will use whatever
the user passes in with a few characters replaced.

This isn't necessarily an issue, but it can lead to extra noise when
running app:update. For example, an app generated with `rails new
MyWebSite` will end up with a configuration line like:

```
config.active_job.queue_name_prefix = "MyWebSite_production"
```

and running app:update will change it to

```
config.active_job.queue_name_prefix = "my_web_site_production"
```

By normalizing app_name when creating an application, the amount of
changes when updating is reduced.

Author: skipkayhil

railties/lib/rails/generators/app_name.rb

@@ -7,7 +7,7 @@ module AppName # :nodoc:
 
       private
         def app_name
-          @app_name ||= original_app_name.tr("\\", "").tr("-. ", "_")
+          @app_name ||= original_app_name.parameterize(preserve_case: true).underscore
         end
 
         def original_app_name

railties/test/generators/app_generator_test.rb

@@ -470,6 +470,12 @@ def test_application_names_are_not_singularized
     assert_file "hats/config/environment.rb", /Rails\.application\.initialize!/
   end
 
+  def test_application_name_is_normalized_in_config
+    run_generator [File.join(destination_root, "MyWebSite"), "-d", "postgresql"]
+    assert_file "MyWebSite/app/views/layouts/application.html.erb", /<title>MyWebSite<\/title>/
+    assert_file "MyWebSite/config/database.yml", /my_web_site_production/
+  end
+
   def test_gemfile_has_no_whitespace_errors
     run_generator
     absolute = File.expand_path("Gemfile", destination_root)