ActiveModel Xml Serialization patch

When passing a :type option to to_xml, the passed type is propagated to all string attributes in the ActiveModel. For instance:

class Person
  include ActiveModel::Serializers::Xml
  extend ActiveModel::Naming

  attr_accessor :attributes
  def initialize(attributes)
    @attributes = attributes
  end
end    

mp = Person.new('foo' => 'string', 'bar' => 1)
puts mp.to_xml(:type => 'Person')

produces:

<?xml version="1.0" encoding="UTF-8"?>
<person type="Person">
  <foo type="Person">string</foo>
  <bar type="integer">1</bar>
</person>

We ran into this because ActiveRecord adds a type option when serializing has_many associations where the association class does not match the association name. For instance:

has_many :cars, :class_name => 'Automobile'

In this case, we wanted the type attribute on each car in the xml, but we don't want it on the car's string attributes :).

Please consider the attached patch for inclusion in the next release of Rails. Thanks.