Use I18n error message for uniqueness validation on Tag#name

This is the first custom error message so a locale file is added. For
any non-en locale, this error message will raise a translation missing
error. Apps that uses this gem should add their own i18n translations.

config/locales/en.yml

@@ -0,0 +1,8 @@
+en:
+  activerecord:
+    errors:
+      models:
+        acts_as_taggable_on/tag:
+          attributes:
+            name:
+              not_unique: has already been taken (using %{type_of_comparison} comparison)

lib/acts_as_taggable_on/tag.rb

@@ -117,9 +117,7 @@ def name_is_unique
       return unless self.class.named(name).exists?
 
       type_of_comparison = ActsAsTaggableOn.strict_case_match ? 'case-sensitive' : 'case-insensitive'
-      error_message = "has already been taken (using #{type_of_comparison} comparison)"
-
-      errors.add(:name, error_message)
+      errors.add(:name, :not_unique, type_of_comparison: type_of_comparison)
     end
 
     class << self

spec/spec_helper.rb

@@ -7,7 +7,10 @@
 require 'logger'
 
 require File.expand_path('../../lib/acts-as-taggable-on', __FILE__)
+
+I18n.load_path << File.expand_path('../../config/locales/en.yml', __FILE__)
 I18n.enforce_available_locales = true
+
 require 'rails'
 require 'rspec/its'
 require 'barrier'