ActiveRecord.to_xml does not :skip_types for included associations

AR.to_xml :skip_types => true doesn't skip_types on included associations:

>> puts Author.find(2).to_xml :skip_types => true

<author>
  <id>2</id>
  <hello-posts type="array">
    <hello-post>
[...]
    </hello-post>
  </hello-post>
</author>

this patch fixes it so this will return:

<author>
  <id>2</id>
  <hello-posts>
    <hello-post>
[...]
    </hello-post>
  </hello-post>
</author>