Replace deprecated MsgpackHandle.AddExt with MessagepackHandle.SetBytesExt #277
Description
MsgpackHandle.AddExt, used here, is deprecated: Use MessagepackHandle.SetBytesExt instead.
Additionally, re-enable TestMsgpackExtensions, in transport/serialize/serializer_test.go, and fix the error is was returning:
serializer_test.go:516: Error registering msgpack extension: cannot modify initialized Handle