[1] The 'status' field has the next options: NEW, READ, DELETED.
So I think that should be used an Enum (opensocial.Enum.messageStatus)
as it's used in another fields when they've several options to choose.
[2] I think that the description is not very clear in the 'type'
field: "The title of the message, specified as an
opensocial.Message.Type"