GenericMessage.unpack should unpack the whole struct, not only its body

[erickvermot]

All the GenericStruct unpacks the whole struct.
Making GenericMessage only unpack the body makes one structure 'protocol' different from the other, for no reason...

PacketIn.unpack() method should be able to unpack the whole message for ex.

I have this ready, and I can make a PR as soon as the changes I've introduced in recent PRs are accepted.

[erickvermot]

the following branch which provides a fix to #361, also fixes this.

master...erickvermot:new_generic_message_work_or_pass