goprotobuf: Don't treat groups as messages.

They no longer get XXX_unrecognized fields, nor satisfy the proto.Message interface.

R=nigeltao, r
CC=golang-dev
http://codereview.appspot.com/6829043
diff --git a/proto/clone_test.go b/proto/clone_test.go
index e6edcf3..5005542 100644
--- a/proto/clone_test.go
+++ b/proto/clone_test.go
@@ -53,6 +53,9 @@
 			Value: []byte("some bytes"),
 		},
 	},
+	Somegroup: &pb.MyMessage_SomeGroup{
+		GroupField: proto.Int32(6),
+	},
 	RepBytes: [][]byte{[]byte("sham"), []byte("wow")},
 }