Allowing to_xml to rename keys in lower camel case format

Hi all,
I’ve created and submitted a patch to resolve an issue I had while attempting to meet the OpenSocial API specifications, which dictates that the xml output have tags formatted as lower camelcase (as opposed to upper camelcase, which is currently the only option). With this patch, simply passing :camelize => true still begets the current behavior, but you can now enable lower camel-casing by passing :camelize => :lower. Please have a look if you get the chance - I appreciate your feedback.

https://rails.lighthouseapp.com/projects/8994/tickets/5903-enable-lower-camel-casing-on-to_xml

Thanks,
Jeremy

Seems good to me. I'll apply it after I verify the tests pass.