Adds missing inflector tests to ensure idempotency

This is a follow up to #4719. It appears that singularize and pluralize
are supposed to be idempotent - i.e. when you call singularize or
pluralize multiple times on the same string, you should get the same
result. (At least for the "officially supported" cases that the stock
inflector is designed to handle.) #4719 added the missing tests for
regular cases, and this commit added the missing tests for the
irregularities.

While I'm at that, I also synced up the irregularity test cases with
the current set of irregularity cases that we ship out-of-the-box.

2 files changed, 12 insertions, 0 deletions

diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index 91ae6bc189..1f32e4ff92 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -413,6 +413,16 @@ class InflectorTest < ActiveSupport::TestCase
     end
   end
 
+  Irregularities.each do |irregularity|
+    singular, plural = *irregularity
+    ActiveSupport::Inflector.inflections do |inflect|
+      define_method("test_singularize_of_irregularity_#{singular}_should_be_the_same") do
+        inflect.irregular(singular, plural)
+        assert_equal singular, ActiveSupport::Inflector.singularize(singular)
+      end
+    end
+  end
+
   [ :all, [] ].each do |scope|
     ActiveSupport::Inflector.inflections do |inflect|
       define_method("test_clear_inflections_with_#{scope.kind_of?(Array) ? "no_arguments" : scope}") do
diff --git a/activesupport/test/inflector_test_cases.rb b/activesupport/test/inflector_test_cases.rb
index 9fa1f417e4..ca4efd2e59 100644
--- a/activesupport/test/inflector_test_cases.rb
+++ b/activesupport/test/inflector_test_cases.rb
@@ -308,5 +308,7 @@ module InflectorTestCases
     'child'  => 'children',
     'sex'    => 'sexes',
     'move'   => 'moves',
+    'cow'    => 'kine',
+    'zombie' => 'zombies',
   }
 end