Add missing require in ActionMailer

Without this, Action Mailer doesn't work like it used to on version 4.
The following snippet fails since version 5:

    > require 'action_mailer'
    > ActionMailer::Base
    NameError: uninitialized constant ActiveSupport::Rescuable

actionmailer/lib/action_mailer.rb

@@ -25,6 +25,7 @@
 require "action_mailer/version"
 
 # Common Active Support usage in Action Mailer
+require "active_support"
 require "active_support/rails"
 require "active_support/core_ext/class"
 require "active_support/core_ext/module/attr_internal"