use I18n.with_locale wherever possible, since it catches errors properly

In particular, pet#load was handling locale-switching itself, but wasn't
switching back to original locale on error. We could've used a rescue
block, but, when I18n.with_locale is so cool, may as well use it fully.

app/helpers/application_helper.rb

@@ -43,10 +43,9 @@ module ApplicationHelper
   end
 
   def canonical_path(resource)
-    original_locale = I18n.locale
-    I18n.locale = I18n.default_locale
+    I18n.with_locale(I18n.default_locale) do
       content_for :meta, tag(:link, :rel => 'canonical', :href => url_for(resource))
-    I18n.locale = original_locale
+    end
   end
 
   def contact_email

app/models/pet.rb

@@ -20,9 +20,7 @@ class Pet < ActiveRecord::Base
     options[:item_scope] ||= Item.scoped
     options[:locale] ||= I18n.default_locale
     
-    original_locale = I18n.locale
-    I18n.locale = options[:locale]
-    
+    I18n.with_locale(options[:locale]) do
       require 'ostruct'
       begin
         neopets_language_code = I18n.neopets_language_code_for(options[:locale])
@@ -60,8 +58,7 @@ class Pet < ActiveRecord::Base
           contents.object_info_registry, contents.object_asset_registry,
           options[:item_scope])
       end
-    
-    I18n.locale = original_locale
+    end
 
     true
   end