Decide on XML-RPC compatibility

XML-RPC is unlike the REST API in that its routes are standardised so we can't change them to allow, for example, multiple authors for a post to be specified when a post is created or edited.

Subsequently I don't think it makes sense to try to expose multiple author information via XML-RPC either. What we do need is to decide exactly how XML-RPC should behave, and ensure it does. It might be that author information in XML-RPC should continue to correlate directly with the `post_author` field.