Replace XML with JSON serialization across the test suite

This allows us to remove the dependency on the XML serializer provided
by the external `activemodel-serializers-xml` gem, and eliminates the
following deprecation warning:

    DEPRECATION WARNING: ActiveModel::Errors#to_xml is deprecated and
    will be removed in Rails 6.2.

Please note: this does not mean Devise doesn't support XML, it simply
means our test suite will use JSON to test non-navigatable formats
instead of XML, for simplicity. Devise's job is not to test object
serialization, so as long as your objects properly serialize to
XML/JSON/any other format, it should work out of the box.

test/rails_app/app/active_record/user.rb

@@ -5,7 +5,6 @@ require 'shared_user'
 class User < ActiveRecord::Base
   include Shim
   include SharedUser
-  include ActiveModel::Serializers::Xml if Devise::Test.rails5_and_up?
 
   validates :sign_in_count, presence: true
 

test/rails_app/app/controllers/users_controller.rb

@@ -3,7 +3,8 @@
 class UsersController < ApplicationController
   prepend_before_action :current_user, only: :exhibit
   before_action :authenticate_user!, except: [:accept, :exhibit]
-  respond_to :html, :xml
+  clear_respond_to
+  respond_to :html, :json
 
   def index
     user_session[:cart] = "Cart"