Fixing abstract class method signatures in message objects

[JesseObrien]

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:
https://github.com/Nexmo/nexmo-php/blob/master/src/Entity/EntityInterface.php#L16

[tjlytle]

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.