goprotobuf: Handle XXX_unrecognized correctly.

In particular,
  - add it to marshaled messages
  - examine it in Clone, Equal

R=r
CC=golang-dev
http://codereview.appspot.com/6449091
diff --git a/proto/equal.go b/proto/equal.go
index f68ffcc..8c26e75 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -126,7 +126,11 @@
 		}
 	}
 
-	// TODO: Deal with XXX_unrecognized.
+	u1 := v1.FieldByName("XXX_unrecognized").Bytes()
+	u2 := v2.FieldByName("XXX_unrecognized").Bytes()
+	if !bytes.Equal(u1, u2) {
+		return false
+	}
 
 	return true
 }