contact.rb

class Contact
  include ActiveModel::Validations
  include ActiveModel::Serialization # should be => include ActiveModel::Serializers::Xml

  validates_presence_of :email, :subject, :body
  validates :email, :format => {
    :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i }
  
  attr_accessor :email
  attr_accessor :subject
  attr_accessor :body

  def initialize(email, subject, body)
    @email = email
    @subject = subject
    @body = body
  end

  def attributes
    @attributes ||= { 'email' => 'nil', 'subject' => 'nil', 'body' => 'nil' }
  end
end

gistfile2.txt

According to step #2 in the extra credit section of the README for lab4, we need to include
ActiveModel::Serialization; but the file contact.rb above throws the following error when 
running rake test:extra

  1) Error:
test_should_be_serializable_to_xml(ContactTest):
NoMethodError: undefined method `to_xml' for #<Contact:0x103438a60>
    /test/extra/contact_test.rb:15:in `test_should_be_serializable_to_xml'

If I change line #3 to 

include ActiveModel::Serializers::Xml

then all tests pass.