Fix 18n fallback when "mount_at_root" is false (#2602)

* Fix fallback locale with "mount_at_root: false"

ruby-i18n does not add the default_locale to the list of fallbacks
anymore since v1.1.0. This breaks fallbacks when "mount_at_root"
config options is set to "false". Fixed this by explicitly adding
the default_locale to the list of fallbacks.

* Added translation fallbacks test with "mount_at_root: false"

---------

Co-authored-by: phernandez <pedrohdezmlg@gmail.com>

middleman-core/features/i18n_builder.feature

@@ -123,8 +123,10 @@ Feature: i18n Builder
       | hello.html                                    |
     And the file "en/index.html" should contain "Howdy"
     And the file "en/hello.html" should contain "Hello World"
+    And the file "en/fallback.html" should contain "Fallback"
     And the file "es/index.html" should contain "Como Esta?"
     And the file "es/hola.html" should contain "Hola World"
+    And the file "es/fallback.html" should contain "Fallback"
 
   Scenario: Running localize with the subset config
     Given a fixture app "i18n-test-app"

middleman-core/fixtures/i18n-test-app/locales/en.yml

@@ -8,3 +8,4 @@ en:
   greetings: "Howdy"
   hi: "Hello"
   tac: "Terms & Conditions"
+  fallback: "Fallback"

middleman-core/fixtures/i18n-test-app/source/localizable/fallback.html.erb

@@ -0,0 +1 @@
+<%= I18n.t(:fallback) %>

middleman-core/lib/middleman-core/core_extensions/i18n.rb

@@ -309,7 +309,7 @@ def configure_i18n
     ::I18n.default_locale = @mount_at_root
 
     # Reset fallbacks to fall back to our new default
-    ::I18n.fallbacks = ::I18n::Locale::Fallbacks.new if ::I18n.respond_to?(:fallbacks)
+    ::I18n.fallbacks = ::I18n::Locale::Fallbacks.new(::I18n.default_locale) if ::I18n.respond_to?(:fallbacks)
   end
 
   Contract ArrayOf[Symbol]