Don't use a class_attribute for ActionMailer::Base.preview_path

Since preview_path is read from ActionMailer::Base when previewing, subclasses
can’t change it so don’t there's no need for the extra overhead imposed by using it.

Author: pixeltrix

actionmailer/lib/action_mailer/preview.rb

@@ -9,7 +9,7 @@ module Previews #:nodoc:
       #
       #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
       #
-      class_attribute :preview_path, instance_writer: false
+      mattr_accessor :preview_path, instance_writer: false
 
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
@@ -85,7 +85,7 @@ def preview_name
 
       protected
         def load_previews #:nodoc:
-          if preview_path?
+          if preview_path
             Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
           end
         end
@@ -94,10 +94,6 @@ def preview_path #:nodoc:
           Base.preview_path
         end
 
-        def preview_path? #:nodoc:
-          Base.preview_path?
-        end
-
         def inform_preview_interceptors(message) #:nodoc:
           Base.preview_interceptors.each do |interceptor|
             interceptor.previewing_email(message)

actionmailer/lib/action_mailer/railtie.rb

@@ -46,7 +46,7 @@ class Railtie < Rails::Railtie # :nodoc:
     end
 
     config.after_initialize do
-      if ActionMailer::Base.preview_path?
+      if ActionMailer::Base.preview_path
         ActiveSupport::Dependencies.autoload_paths << ActionMailer::Base.preview_path
       end
     end