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.