add block passthrough to devise_mail

ActionMailer's ``mail`` method may receive a block for customizing the mails
format
``devise_mail`` now has the same functionality by just
passing the block to ``mail`` call.

fixes plataformatec/devise#2341

lib/devise/mailers/helpers.rb

@@ -11,9 +11,9 @@ module Devise
       protected
 
       # Configure default email options
-      def devise_mail(record, action, opts={})
+      def devise_mail(record, action, opts = {}, &block)
         initialize_from_record(record)
-        mail headers_for(action, opts)
+        mail headers_for(action, opts), &block
       end
 
       def initialize_from_record(record)