emails: fix exception when disabling dmail notifications.

Fix an `undefined method 'find' for Maintenance::User:Module` exception
when disabling email notifications using the "Disable notifications"
link in dmails.

app/controllers/maintenance/user/email_notifications_controller.rb

@@ -5,6 +5,8 @@ module Maintenance
     class EmailNotificationsController < ApplicationController
       class VerificationError < StandardError; end
 
+      respond_to :html, :json, :xml
+
       before_action :validate_sig, :only => [:destroy]
       rescue_from VerificationError, with: :render_verification_error
 
@@ -12,9 +14,9 @@ module Maintenance
       end
 
       def destroy
-        @user = User.find(params[:user_id])
+        @user = ::User.find(params[:user_id])
-        @user.receive_email_notifications = false
+        @user.update!(receive_email_notifications: false)
-        @user.save
+        respond_with(@user)
       end
 
       private

test/functional/maintenance/user/email_notifications_controller.rb

@@ -0,0 +1,38 @@
+require "test_helper"
+
+module Maintenance
+  module User
+    class EmailNotificationsControllerTest < ActionDispatch::IntegrationTest
+      context "Email notifications" do
+        setup do
+          @user = create(:user, receive_email_notifications: true)
+          @verifier = ActiveSupport::MessageVerifier.new(Danbooru.config.email_key, digest: "SHA256", serializer: JSON)
+          @sig = @verifier.generate(@user.id.to_s)
+        end
+
+        context "#show" do
+          should "render" do
+            get_auth maintenance_user_email_notification_path(user_id: @user.id), @user
+            assert_response :success
+          end
+        end
+
+        context "#destroy" do
+          should "disable email notifications" do
+            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: @sig), @user
+
+            assert_response :success
+            assert_equal(false, @user.reload.receive_email_notifications)
+          end
+
+          should "not disable email notifications when given an incorrect signature" do
+            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: "foo"), @user
+
+            assert_response 403
+            assert_equal(true, @user.reload.receive_email_notifications)
+          end
+        end
+      end
+    end
+  end
+end