Eager autoload mail gem when eager load is true (#32808)

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

actionmailer/CHANGELOG.md

+*   Ensure mail gem is eager autoloaded when eager load is true to prevent thread deadlocks.
+
+    *Samuel Cochran*
+
+
 ## Rails 5.2.0 (April 09, 2018) ##
 
 *   Bring back proc with arity of 1 in `ActionMailer::Base.default` proc

actionmailer/lib/action_mailer.rb

@@ -52,6 +52,13 @@ module ActionMailer
   autoload :TestHelper
   autoload :MessageDelivery
   autoload :DeliveryJob
+
+  def self.eager_load!
+    super
+
+    require "mail"
+    Mail.eager_autoload!
+  end
 end
 
 autoload :Mime, "action_dispatch/http/mime_type"