According to https://github.com/blongden/vnd.error, logref is for "expressing a (numeric/alpha/alphanumeric) identifier". This patches `VndError` to support both strings and integers, ensuring each serializes properly. NOTE: `VndErrors` has been deprecated due to the spec itself being dead since 2014. However, it must be supported until fully removed from Spring HATEOAS. Original pull request: #1293.