Update default navigation and menu button labels (#464)

These had fallen behind govuk-frontend, this change brings them back in
line. Thank you for spotting @lfdebrux!

Fixes #460

lib/govuk/components/engine.rb

@@ -87,8 +87,8 @@ def reset!
       default_exit_this_page_timed_out_text: nil,
       default_exit_this_page_press_two_more_times_text: nil,
       default_exit_this_page_press_one_more_time_text: nil,
-      default_header_navigation_label: 'Navigation menu',
-      default_header_menu_button_label: 'Show or hide navigation menu',
+      default_header_navigation_label: 'Menu',
+      default_header_menu_button_label: 'Show or hide menu',
       default_header_logotype: 'GOV.UK',
       default_header_homepage_url: '/',
       default_header_service_name: nil,

spec/components/govuk_component/header_component_spec.rb

@@ -204,6 +204,10 @@
         expect(rendered_content).to have_tag('nav')
       end
 
+      specify 'nav element contains default aria label' do
+        expect(rendered_content).to have_tag('nav', with: { 'aria-label' => 'Menu' })
+      end
+
       specify 'nav contains the right number of items' do
         expect(rendered_content).to have_tag("li", with: { class: "govuk-header__navigation-item" }, count: navigation_items.count)
       end
@@ -292,7 +296,7 @@
       describe 'menu button (for mobile)' do
         let(:button_text) { 'Menu' }
         let(:button_classes) { %w(govuk-header__menu-button govuk-js-header-toggle) }
-        let(:button_aria_label) { 'Show or hide navigation menu' }
+        let(:button_aria_label) { 'Show or hide menu' }
 
         specify 'the button is rendered' do
           expect(rendered_content).to have_tag('div', with: { class: 'govuk-header__content' }) do