Don't assume resource param is :id when using shallow routes

Since #5581 added support for resources with custom params we should
not assume that it is :id when using shallow resource routing.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -965,6 +965,8 @@ def member_scope
             "#{path}/:#{param}"
           end
 
+          alias :shallow_scope :member_scope
+
           def new_scope(new_path)
             "#{path}/#{new_path}"
           end
@@ -1509,7 +1511,7 @@ def shallow_scoping? #:nodoc:
 
           def path_for_action(action, path) #:nodoc:
             prefix = shallow_scoping? ?
-              "#{@scope[:shallow_path]}/#{parent_resource.path}/:id" : @scope[:path]
+              "#{@scope[:shallow_path]}/#{parent_resource.shallow_scope}" : @scope[:path]
 
             if canonical_action?(action, path.blank?)
               prefix.to_s

actionpack/test/dispatch/routing_test.rb

@@ -487,6 +487,12 @@ def self.call(params, request)
         resources :messages
       end
 
+      resources :orders do
+        constraints :download => /[0-9a-f]{8}-(?:[0-9a-f]{4}-){3}[0-9a-f]{12}/ do
+          resources :downloads, :param => :download, :shallow => true
+        end
+      end
+
       scope :as => "routes" do
         get "/c/:id", :as => :collision, :to => "collision#show"
         get "/collision", :to => "collision#show"
@@ -2254,6 +2260,12 @@ def test_custom_param_constraint
     assert_equal 404, @response.status
   end
 
+  def test_shallow_custom_param
+    get '/downloads/0c0c0b68-d24b-11e1-a861-001ff3fffe6f.zip'
+    assert_equal 'downloads#show', @response.body
+    assert_equal '0c0c0b68-d24b-11e1-a861-001ff3fffe6f', @request.params[:download]
+  end
+
 private
   def with_https
     old_https = https?