Fixed bug where NoIndent broke KeyAttr (reported by David Haas)
Added copy_hash helper method which may be overridden to avoid tied hashes becoming untied during XMLout (patch from Jan Sundberg)
Fixed bug where GroupTags corrupted source hashref in XMLout (reported by Bram)
Tweaks to SuppressEmpty for undef with XMLout (report from jamesb), behaviour now matches docs and additional behaviour of setting option to 1 will skip undefined values altogether