Fix rails g devise:views -v mailer with the Simple Form generator.

Our Simple Form generator shouldn't be responsible for generating the `mailer`
view directory, so we should skip it and let the Erb generator do the job.

Closes #3254.

lib/generators/devise/views_generator.rb

@@ -83,6 +83,13 @@ module Devise
       source_root File.expand_path("../../templates/simple_form_for", __FILE__)
       desc "Copies simple form enabled views to your application."
       hide!
+
+      def copy_views
+        if options[:views]
+          options[:views].delete('mailer')
+        end
+        super
+      end
     end
 
     class ErbGenerator < Rails::Generators::Base #:nodoc:

test/generators/views_generator_test.rb

@@ -46,6 +46,13 @@ class ViewsGeneratorTest < Rails::Generators::TestCase
     assert_no_file "app/views/devise/mailer/confirmation_instructions.html.erb"
   end
 
+  test "Assert mailer specific directory with simple form" do
+    run_generator %w(-v mailer -b simple_form_for)
+    assert_file "app/views/devise/mailer/confirmation_instructions.html.erb"
+    assert_file "app/views/devise/mailer/reset_password_instructions.html.erb"
+    assert_file "app/views/devise/mailer/unlock_instructions.html.erb"
+  end
+
   test "Assert specified directories with scope" do
     run_generator %w(users -v sessions)
     assert_file "app/views/users/sessions/new.html.erb"