Mounted Rack apps should have default named routes based on app name

This fixes a regression in 4.2.0 from 4.1.8.

https://github.com/rails/rails/pull/17823 fixed a similar regression regarding _explicitly_ named routes for a mounted Rack app, but there was another regression for the default value.

With a route like:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route'
    end

The "Prefix" column of rake routes gives the following:

- 4.1.8:         mountable_web
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     some_route   <- regression

This fixes the default to go back to being based off the name of the class like the docs specify: https://github.com/rails/rails/blob/785d04e3109f69d0b9b9f4732179592f0ef04e52/actionpack/lib/action_dispatch/routing/mapper.rb#L558-L560

Explicitly named routes still work correctly per https://github.com/rails/rails/pull/17823:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route', as: 'named'
    end

- 4.1.8:         named
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     named

1 files changed, 10 insertions, 3 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 5040aa82b2..f07a4aa674 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -579,15 +579,14 @@ module ActionDispatch
 
           raise "A rack application must be specified" unless path
 
-          rails_app = rails_app? app
-          options[:as] ||= app.railtie_name if rails_app
+          options[:as] ||= app_name(app)
 
           target_as       = name_for_action(options[:as], path)
           options[:via] ||= :all
 
           match(path, options.merge(:to => app, :anchor => false, :format => false))
 
-          define_generate_prefix(app, target_as) if rails_app
+          define_generate_prefix(app, target_as) if rails_app?(app)
           self
         end
 
@@ -612,6 +611,14 @@ module ActionDispatch
             app.is_a?(Class) && app < Rails::Railtie
           end
 
+          def app_name(app)
+            if rails_app?(app)
+              app.railtie_name
+            elsif class_name = app.try(:name)
+              ActiveSupport::Inflector.underscore(class_name).tr("/", "_")
+            end
+          end
+
           def define_generate_prefix(app, name)
             _route = @set.named_routes.get name
             _routes = @set