makes the link_to controller method available in the view.
# File actionmailer/lib/action_mailer/helpers.rb, line 73
def helper_method(*methods)
methods.flatten.each do |method|
master_helper_module.module_eval "def \#{method}(*args, &block)\ncontroller.__send__(%(\#{method}), *args, &block)\nend\n"
end
end
