http://d.hatena.ne.jp/apo-co/20090928/p1
include ApplicationHelper
helper :application
の宣言を追加する。
http://stackoverflow.com/questions/4937208/access-helpers-from-mailer
http://stackoverflow.com/questions/1416978/how-to-use-my-view-helpers-in-my-actionmailer-views
class UserMailer < ActionMailer::Base include ApplicationHelper helper :application end