allow preview interceptors to be registered through `config.action_mailer`.

This was partially broken because `preview_interceptors=` just assigned the
raw values, whithout going through `register_preview_interceptor`. Now the
Action Mailer railtie takes care of the `preview_interceptors` option.

This commit is a partial revert of:

Revert "Merge pull request #15739 from y-yagi/correct_doc_for_action_mailer_base"

This reverts commit a15704d7f35f17d34d0118546799141d6f853656, reversing
changes made to 1bd12a8609d275ad75fcc4b622ca4f5b32dc76be.

/cc @kuldeepaggarwal @y-yagi

1 files changed, 32 insertions, 0 deletions

diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index e95c3fa20d..207a0c7e86 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -8,6 +8,12 @@ end
 
 class ::MyOtherMailInterceptor < ::MyMailInterceptor; end
 
+class ::MyPreviewMailInterceptor
+  def self.previewing_email(email); email; end
+end
+
+class ::MyOtherPreviewMailInterceptor < ::MyPreviewMailInterceptor; end
+
 class ::MyMailObserver
   def self.delivered_email(email); email; end
 end
@@ -460,6 +466,32 @@ module ApplicationTests
       assert_equal [::MyMailInterceptor, ::MyOtherMailInterceptor], ::Mail.send(:class_variable_get, "@@delivery_interceptors")
     end
 
+    test "registers preview interceptors with ActionMailer" do
+      add_to_config <<-RUBY
+        config.action_mailer.preview_interceptors = MyPreviewMailInterceptor
+      RUBY
+
+      require "#{app_path}/config/environment"
+      require "mail"
+
+      _ = ActionMailer::Base
+
+      assert_equal [::MyPreviewMailInterceptor], ActionMailer::Base.preview_interceptors
+    end
+
+    test "registers multiple preview interceptors with ActionMailer" do
+      add_to_config <<-RUBY
+        config.action_mailer.preview_interceptors = [MyPreviewMailInterceptor, "MyOtherPreviewMailInterceptor"]
+      RUBY
+
+      require "#{app_path}/config/environment"
+      require "mail"
+
+      _ = ActionMailer::Base
+
+      assert_equal [MyPreviewMailInterceptor, MyOtherPreviewMailInterceptor], ActionMailer::Base.preview_interceptors
+    end
+
     test "registers observers with ActionMailer" do
       add_to_config <<-RUBY
         config.action_mailer.observers = MyMailObserver