Allow passing interpolations to `#default_i18n_subject`, e.g.:

    # config/locales/en.yml
    en:
      user_mailer:
        welcome:
          subject: 'Hello, %{username}'

    # app/mailers/user_mailer.rb
    class UserMailer < ActionMailer::Base
      def welcome(user)
        mail(subject: default_i18n_subject(username: user.name))
      end
    end

1 files changed, 4 insertions, 0 deletions

diff --git a/actionmailer/test/mailers/base_mailer.rb b/actionmailer/test/mailers/base_mailer.rb
index 8fca6177bd..504ca36483 100644
--- a/actionmailer/test/mailers/base_mailer.rb
+++ b/actionmailer/test/mailers/base_mailer.rb
@@ -123,4 +123,8 @@ class BaseMailer < ActionMailer::Base
     mail(:template_name => "welcome")
     nil
   end
+
+  def with_subject_interpolations
+    mail(subject: default_i18n_subject(rapper_or_impersonator: 'Slim Shady'), body: '')
+  end
 end