This is broken in the current @beta composer release. All of these classes will break (read: FATAL error) when being used because they don't implement the abstract method signature properly.
Method signature declaration here:
Fixing abstract class method signatures in message objects
Thanks, this was a bit of a refactor from an earlier concept, so had not paid enough attention. Will merge, should also mention the next release will change some of the underlying request / response concepts. Hope your use was just to get it to work, and not utilizing the underlying request / response data.