Add support for optional root segments containing slashes

Optional segments with a root scope need to have the leading slash
outside of the parentheses, otherwise the generated url will be empty.
However if the route has non-optional elements then the leading slash
needs to remain inside the parentheses otherwise the generated url
will have two leading slashes, e.g:

Blog::Application.routes.draw do
  get '/(:category)', :to => 'posts#index', :as => :root
  get '/(:category)/author/:name', :to => 'posts#author', :as => :author
end

$ rake routes
  root GET /(:category)(.:format)              posts#index
author GET (/:category)/author/:name(.:format) posts#author

This change adds support for optional segments that contain a slash,
allowing support for urls like /page/2 for the root path, e.g:

Blog::Application.routes.draw do
  get '/(page/:page)', :to => 'posts#index', :as => :root
end

$ rake routes
root GET /(page/:page)(.:format) posts#index

Fixes #7073

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -262,7 +262,7 @@ def defaults_from_constraints(constraints)
       # for root cases, where the latter is the correct one.
       def self.normalize_path(path)
         path = Journey::Router::Utils.normalize_path(path)
-        path.gsub!(%r{/(\(+)/?}, '\1/') unless path =~ %r{^/\(+[^/]+\)$}
+        path.gsub!(%r{/(\(+)/?}, '\1/') unless path =~ %r{^/\(+[^)]+\)$}
         path
       end
 

actionpack/test/dispatch/routing_test.rb

@@ -2679,3 +2679,30 @@ def show
     end
   end
 end
+
+class TestOptionalRootSegments < ActionDispatch::IntegrationTest
+  stub_controllers do |routes|
+    Routes = routes
+    Routes.draw do
+      get '/(page/:page)', :to => 'pages#index', :as => :root
+    end
+  end
+
+  def app
+    Routes
+  end
+
+  include Routes.url_helpers
+
+  def test_optional_root_segments
+    get '/'
+    assert_equal 'pages#index', @response.body
+    assert_equal '/', root_path
+
+    get '/page/1'
+    assert_equal 'pages#index', @response.body
+    assert_equal '1', @request.params[:page]
+    assert_equal '/page/1', root_path('1')
+    assert_equal '/page/1', root_path(:page => '1')
+  end
+end