`mail()` without arguments is a getter for the current mail.

This behavior is documented in our guides (http://edgeguides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks)
but was broken in the past. This commit short curcuits
the `mail` method if:

  1. mail() was previously called
  2. no headers are passed
  3. no block is passed

Closes #13090.

/cc @pixeltrix

1 files changed, 19 insertions, 0 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index dc8c6bdf74..857cde399a 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,22 @@
+*   Calling `mail()` without arguments serves as getter for the current mail
+    message and keeps previously set headers.
+
+    Example:
+
+        class MailerWithCallback < ActionMailer::Base
+          after_action :a_callback
+
+          def welcome
+            mail subject: "subject", to: ["joe@example.com"]
+          end
+
+          def a_callback
+            mail # => returns the current mail message
+          end
+        end
+
+    *Yves Senn*
+
 *   Instrument the generation of Action Mailer messages. The time it takes to
     generate a message is written to the log.