let demodulize do less work, and add tests

This is also faster on 1.9.

activesupport/lib/active_support/inflector/methods.rb

@@ -166,7 +166,9 @@ def dasherize(underscored_word)
     #   "ActiveRecord::CoreExtensions::String::Inflections".demodulize # => "Inflections"
     #   "Inflections".demodulize                                       # => "Inflections"
     def demodulize(class_name_in_module)
-      class_name_in_module.to_s.gsub(/^.*::/, '')
+      # If you remove the module part of an empty string, you get an empty string.
+      # That's why the regexp uses the * quantifier.
+      class_name_in_module.to_s[/[^:]*\z/]
     end
 
     # Creates a foreign key name from a class name.

activesupport/test/inflector_test.rb

@@ -194,6 +194,8 @@ def test_underscore_with_slashes
 
   def test_demodulize
     assert_equal "Account", ActiveSupport::Inflector.demodulize("MyApplication::Billing::Account")
+    assert_equal "Account", ActiveSupport::Inflector.demodulize("Account")
+    assert_equal "", ActiveSupport::Inflector.demodulize("")
   end
 
   def test_foreign_key