Integrate ActiveJob / DeliverLater / GlobalID with Rails

WIP.
I think We can make it work easily.

First we need to move this definition of `method_missing` to a new module that we will included in the singleton class of `ActionMailer::Base`. After we only need to include the `ActiveJob::DeliverLater` module on `ActionMailer::Base.singleton_class`.

Here a pseudo-implementation:

```diff
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index df0c945..7598337 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -547,17 +547,20 @@ module ActionMailer
         payload[:mail]       = mail.encoded
       end
 
-      def method_missing(method_name, *args) # :nodoc:
-        if respond_to?(method_name)
-          if defined?(::ActiveJob) && action_methods.include?(method_name.to_s)
-            DeliverLater::MailMessageWrapper.new(self, method_name, *args)
-          else
+      module Foo
+        protected
+
+        def method_missing(method_name, *args) # :nodoc:
+          p "Not: #{method_name}"
+          if respond_to?(method_name)
             new(method_name, *args).message
+          else
+            super
           end
-        else
-          super
         end
       end
+
+      include Foo
     end
 
     attr_internal :message
diff --git a/actionmailer/lib/action_mailer/deliver_later.rb b/actionmailer/lib/action_mailer/deliver_later.rb
index 5609e35..92a8e92 100644
--- a/actionmailer/lib/action_mailer/deliver_later.rb
+++ b/actionmailer/lib/action_mailer/deliver_later.rb
@@ -5,5 +5,18 @@ module ActionMailer
     extend ActiveSupport::Autoload
     autoload :Job
     autoload :MailMessageWrapper
+
+    module Mixin
+
+      protected
+
+      def method_missing(method_name, *args)
+        if respond_to?(method_name)
+          DeliverLater::MailMessageWrapper.new(self, method_name, *args)
+        else
+          super
+        end
+      end
+    end
   end
-end
\ No newline at end of file
+end
diff --git a/actionmailer/test/deliver_later_test.rb b/actionmailer/test/deliver_later_test.rb
index 829c968..602e597 100644
--- a/actionmailer/test/deliver_later_test.rb
+++ b/actionmailer/test/deliver_later_test.rb
@@ -5,6 +5,8 @@ require 'abstract_unit'
 require 'minitest/mock'
 require_relative 'mailers/delayed_mailer'
 
+ActionMailer::Base.singleton_class.send(:include, ActionMailer::DeliverLater::Mixin)
+
 class MailerTest < ActiveSupport::TestCase
 
   setup do
```