[Patch] ActiveRecord's XmlSerializer should not singularize already singular association names

The current to_xml behavior is incorrect. It singularizes already singular association names, resulting in xml that does not represent the original models/associations. Attached patch fixes this.

Steps to reproduce

Say we have the following models:

class Address < ActiveRecord::Base; belongs_to :person; end
class Person < ActiveRecord::Base; has_one :address; end

Now, in Rails console, find a Person and do:

> @person.to_xml(:skip_instruct => true, :include => :address)

Current to_xml result (Rails 3.0.4):

<person>
  <id type="integer">1</id>
  <addres>
    <id type="integer">1</id>
  </addres>
</person>

After applying patch:

<person>
  <id type="integer">1</id>
  <address>
    <id type="integer">1</id>
  </address>
</person>

(notice the difference in the spelling of 'address')