fix pluralize behavior for words that consist of non-ascii characters

The code below returns "猫" in 3.2, but "猫s" in 4.0.
```ruby
ActiveSupport::Inflector.inflections do |inflect|
  inflect.uncountable "猫"
end
"猫".pluralize
```

2 files changed, 10 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index be369d21c6..cb36a16a51 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -372,7 +372,7 @@ module ActiveSupport
     def apply_inflections(word, rules)
       result = word.to_s.dup
 
-      if word.empty? || inflections.uncountables.include?(result.downcase[/\b\w+\Z/])
+      if word.empty? || inflections.uncountables.include?(result.downcase[/\b\p{Word}+\Z/])
         result
       else
         rules.each { |(rule, replacement)| break if result.sub!(rule, replacement) }
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index a0764f6d6b..56bc6bb99f 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -1,3 +1,4 @@
+# encoding: utf-8
 require 'abstract_unit'
 require 'active_support/inflector'
 
@@ -31,6 +32,14 @@ class InflectorTest < ActiveSupport::TestCase
     assert_equal "", ActiveSupport::Inflector.pluralize("")
   end
 
+  def test_pluralize_for_words_with_non_ascii_characters
+    ActiveSupport::Inflector.inflections do |inflect|
+      inflect.uncountable "猫"
+    end
+    assert_equal "猫", ActiveSupport::Inflector.pluralize("猫")
+    ActiveSupport::Inflector.inflections.uncountables.pop
+  end
+
   ActiveSupport::Inflector.inflections.uncountable.each do |word|
     define_method "test_uncountability_of_#{word}" do
       assert_equal word, ActiveSupport::Inflector.singularize(word)