Test cloning tx after rx dropped (servo#377)

The test transfer_closed_sender will fail if issue 13 regresses, but
this explicit test documents the expected behaviour better.

Ref: servo#13

src/test.rs

@@ -688,6 +688,13 @@ fn test_reentrant() {
     assert_eq!(null, receiver.recv().unwrap());
 }
 
+#[test]
+fn clone_sender_after_receiver_dropped() {
+    let (tx, rx) = ipc::channel::<u32>().unwrap();
+    drop(rx);
+    let _tx2 = tx.clone();
+}
+
 #[test]
 fn transfer_closed_sender() {
     let (main_tx, main_rx) = ipc::channel().unwrap();