Enable lower camel-casing on to_xml

While the to_xml method on activemodel implementations and in the core extensions does allow you to pass :camelize => true as an option, it defaults to upper camel case and allows no alternative. I propose keeping the default behavior, but allowing it to accept :camelize => :lower to indicate that the tag names should be represented as lower camel-case, which is a requirement for certain XML specifications (e.g. OpenSocial XML format).