Allow parent_mailer to be customizable via Devise.parent_mailer, useful for engines

app/mailers/devise/mailer.rb

@@ -1,4 +1,4 @@
-class Devise::Mailer < ::ActionMailer::Base
+class Devise::Mailer < Devise.parent_mailer.constantize
   include Devise::Mailers::Helpers
 
   def confirmation_instructions(record, opts={})

lib/devise.rb

@@ -199,6 +199,12 @@ module Devise
   mattr_accessor :parent_controller
   @@parent_controller = "ApplicationController"
 
+  # The parent mailer all Devise mailers inherit from.
+  # Defaults to ActionMailer::Base. This should be set early
+  # in the initialization process and should be set to a string.
+  mattr_accessor :parent_mailer
+  @@parent_mailer = "ActionMailer::Base"
+
   # The router Devise should use to generate routes. Defaults
   # to :main_app. Should be overriden by engines in order
   # to provide custom routes.