Fix loading issue of ActionMailer::MessageDelivery

[yujideveloper]

I had an error NameError (uninitialized constant #<Class:ActionMailer::Base>::MessageDelivery) that is difficult to reproduce.
I'm sending some mails in some action of ActionController using ActionMailer.

I think this is concurrency issue that is occurred when using thread-base app server like Puma.

This occurs in Rails 5.0.7 and earlier, but not occurs in Rails 5.1.0 and later.
I was using Rails 5.0.6 and Ruby 2.3.1 when this is occurred.

Before 5.0.7, ActionMailer::MessageDelivery is only used in ActionMailer::Base#method_missing.
So, ActionMailer::MessageDelivery will be loaded when mail method is called.
https://github.com/rails/rails/blob/v5.0.7/actionmailer/lib/action_mailer.rb#L50
https://github.com/rails/rails/blob/v5.0.7/actionmailer/lib/action_mailer/base.rb#L563

After 5.1.0, ActionMailer::MessageDelivery is used in the module definition of ActionMailer::Parameterized inclueded in ActionMailer::Base.
So, ActionMailer::MessageDelivery will be loaded when defining a class that inherited ActionMailer::Base.
https://github.com/rails/rails/blob/v5.1.0/actionmailer/lib/action_mailer.rb#L51
https://github.com/rails/rails/blob/v5.1.0/actionmailer/lib/action_mailer/base.rb#L438
https://github.com/rails/rails/blob/v5.1.0/actionmailer/lib/action_mailer/parameterized.rb#L122

I think this is similar to #27849.

Thanks.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @rafaelfranca (or someone else) soon.

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[rails-bot]

Warning

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[rafaelfranca]

5.0 is not maintained anymore for bug fixes. 5.0.7 was the last release.

[rafaelfranca]

Sorry, we still have some core members maintaining that version. I'll merge this PR.