Cannot specify detach state field when using inheritance

During ClassMetaData.validateDetachable() a call is made to getDetachedStateField() which returns the closest defined detached state field. This field is simply checked for not null, which causes all subclasses to take the parent-detach-state exception path.

getDetachedStateField() sets _detachState as a side effect, so either this call should be avoided and the _detachState field checked directly, or the returned field's defining class should be checked against _type depending on if the side effect is required or not.