More consistent ID printing

Use unsigned rather than signed for IDs, so they match up with what we
see in other prints.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>

tests/connection-test.c

@@ -236,7 +236,7 @@ TEST(connection_marshal)
 	marshal(&data, "n", 12, &object);
 	assert(data.buffer[2] == object.id);
 
-	marshal(&data, "n", 12, NULL);
+	marshal(&data, "?n", 12, NULL);
 	assert(data.buffer[2] == 0);
 
 	array.data = (void *) text;