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 a15704d7, reversing
changes made to 1bd12a86.

/cc @kuldeepaggarwal @y-yagi

actionmailer/CHANGELOG.md

+*   Allow preview interceptors to be registered through
+    `config.action_mailer.preview_interceptors`.
+
+    See #15739.
+
+    *Yves Senn*
+
 Please check [4-1-stable](https://github.com/rails/rails/blob/4-1-stable/actionmailer/CHANGELOG.md) for previous changes.

actionmailer/lib/action_mailer/base.rb

@@ -340,7 +340,7 @@ module ActionMailer
   #     end
   #   end
   #
-  #   ActionMailer::Base.register_preview_interceptor :css_inline_styler
+  #   config.action_mailer.preview_interceptors :css_inline_styler
   #
   # Note that interceptors need to be registered both with <tt>register_interceptor</tt>
   # and <tt>register_preview_interceptor</tt> if they should operate on both sending and

actionmailer/lib/action_mailer/railtie.rb

@@ -33,6 +33,7 @@ class Railtie < Rails::Railtie # :nodoc:
         include app.routes.mounted_helpers
 
         register_interceptors(options.delete(:interceptors))
+        register_preview_interceptors(options.delete(:preview_interceptors))
         register_observers(options.delete(:observers))
 
         options.each { |k,v| send("#{k}=", v) }

railties/test/application/configuration_test.rb

@@ -8,6 +8,12 @@ def self.delivering_email(email); email; 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 @@ def index
       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