Delegate mailer preview file creation to test_unit generator

Author: joaquintomas2003

railties/lib/rails/generators/rails/authentication/authentication_generator.rb

@@ -30,8 +30,6 @@ def create_authentication_files
 
           template "app/views/passwords_mailer/reset.html.erb"
           template "app/views/passwords_mailer/reset.text.erb"
-
-          template "test/mailers/previews/passwords_mailer_preview.rb"
         end
       end
 

railties/lib/rails/generators/test_unit/authentication/authentication_generator.rb

@@ -15,6 +15,10 @@ def create_controller_test_files
         template "test/controllers/passwords_controller_test.rb"
       end
 
+      def create_mailer_preview_files
+        template "test/mailers/previews/passwords_mailer_preview.rb" if defined?(ActionMailer::Railtie)
+      end
+
       def create_test_helper_files
         template "test/test_helpers/session_test_helper.rb"
       end