Fix assertion that was never run

In order to run whether the `welcome` method of the ActionMailer::Base
subclass raises an error, `message` must be called, otherwise the method
is not executed at all.

You could just replace with `def welcome; raise StandardError; end` and you
would still see a passing test.

This commit fixes the test so the assertion is actually executed, just like
any other tests in the file, where `.message` is called.

actionmailer/test/base_test.rb

@@ -287,7 +287,7 @@ class BaseTest < ActiveSupport::TestCase
       end
     end
 
-    assert_nothing_raised { LateAttachmentAccessorMailer.welcome }
+    assert_nothing_raised { LateAttachmentAccessorMailer.welcome.message }
   end
 
   # Implicit multipart