debug_missing_translation configuration added to action_view

`I18n.translate` helper will wrap the missing translation keys
in a <span> tag only if `debug_missing_translation` configuration has
a truthy value. Default value is `true`. For example in `application.rb`:

    # in order to turn off missing key wrapping
    config.action_view.debug_missing_translation = false

1 files changed, 10 insertions, 0 deletions

diff --git a/actionview/lib/action_view/helpers/translation_helper.rb b/actionview/lib/action_view/helpers/translation_helper.rb
index 4c4d2c4457..152e1b1211 100644
--- a/actionview/lib/action_view/helpers/translation_helper.rb
+++ b/actionview/lib/action_view/helpers/translation_helper.rb
@@ -6,7 +6,15 @@ module ActionView
   # = Action View Translation Helpers
   module Helpers
     module TranslationHelper
+      extend ActiveSupport::Concern
+
       include TagHelper
+
+      included do
+        mattr_accessor :debug_missing_translation
+        self.debug_missing_translation = true
+      end
+
       # Delegates to <tt>I18n#translate</tt> but also performs three additional
       # functions.
       #
@@ -95,6 +103,8 @@ module ActionView
             title << ", " << interpolations.map { |k, v| "#{k}: #{ERB::Util.html_escape(v)}" }.join(', ')
           end
 
+          return title unless ActionView::Base.debug_missing_translation
+
           content_tag('span', keys.last.to_s.titleize, class: 'translation_missing', title: title)
         end
       end