Revert "store fallbacks in Thread.current"

This reverts commit d024044.

And there is now a test to ensure that this feature does not regress again.

lib/i18n/backend/fallbacks.rb

@@ -10,15 +10,17 @@
 #
 #   I18n::Backend::Simple.include(I18n::Backend::Fallbacks)
 module I18n
+  @@fallbacks = nil
+
   class << self
     # Returns the current fallbacks implementation. Defaults to +I18n::Locale::Fallbacks+.
     def fallbacks
-      Thread.current[:i18n_fallbacks] ||= I18n::Locale::Fallbacks.new
+      @@fallbacks ||= I18n::Locale::Fallbacks.new
     end
 
     # Sets the current fallbacks implementation. Use this to set a different fallbacks implementation.
     def fallbacks=(fallbacks)
-      Thread.current[:i18n_fallbacks] = fallbacks.is_a?(Array) ? I18n::Locale::Fallbacks.new(fallbacks) : fallbacks
+      @@fallbacks = fallbacks.is_a?(Array) ? I18n::Locale::Fallbacks.new(fallbacks) : fallbacks
     end
   end
 

test/backend/fallbacks_test.rb

@@ -107,6 +107,12 @@ def setup
     assert_equal 'default', I18n.t(:missing_bar, count: 1, default: 'default')
     assert_equal 'default', I18n.t(:missing_bar, count: 0, default: 'default')
   end
+
+  test "multi-threaded fallbacks" do
+    Thread.new do
+
+    end
+  end
 end
 
 # See Issue #534
@@ -156,6 +162,25 @@ def setup
   end
 end
 
+# See Issue #546
+class I18nBackendFallbacksLocalizeTestWithMultipleThreads < I18n::TestCase
+  class Backend < I18n::Backend::Simple
+    include I18n::Backend::Fallbacks
+  end
+
+  def setup
+    super
+    I18n.backend = Backend.new
+    I18n.enforce_available_locales = false
+    I18n.fallbacks = [I18n.default_locale]
+    store_translations(:en, time: { formats: { fallback: 'en fallback' } })
+  end
+
+  test "falls back to default locale - Issue #546" do
+    Thread.new { assert_equal 'en fallback', I18n.l(Time.now, format: :fallback, locale: "un-supported") }.join
+  end
+end
+
 
 class I18nBackendFallbacksLocalizeTest < I18n::TestCase
   class Backend < I18n::Backend::Simple