fix(client): detect connection closes as pool tries to use

Currently, if the remote closes the connection at the same time that the
pool selects it to use for a new request, the connection may actually
hang. This fix will now more allow the keep-alive read to check the
socket even when the `Conn` think it's busy.

If the connection was closed before the request write happened, returns
back an `Error::Cancel`, letting the user know they could safely retry
it.

Closes #1439

src/client/dispatch.rs

@@ -85,7 +85,7 @@ impl<T, U> Drop for Receiver<T, U> {
         // - Err: unreachable
         while let Ok(Async::Ready(Some((_val, cb)))) = self.inner.poll() {
             // maybe in future, we pass the value along with the error?
-            let _ = cb.send(Err(::Error::new_canceled()));
+            let _ = cb.send(Err(::Error::new_canceled(None)));
         }
     }