Make cancel_query take a ref

Even though CancelData is Copy it's a bit weird for a function to take
ownership when it doesn't need to.

Author: sfackler

src/lib.rs

@@ -267,11 +267,11 @@ pub struct CancelData {
 /// thread::spawn(move || {
 ///     conn.execute("SOME EXPENSIVE QUERY", &[]).unwrap();
 /// });
-/// postgres::cancel_query(url, SslMode::None, cancel_data).unwrap();
+/// postgres::cancel_query(url, SslMode::None, &cancel_data).unwrap();
 /// ```
 pub fn cancel_query<T>(params: T,
                        ssl: SslMode,
-                       data: CancelData)
+                       data: &CancelData)
                        -> result::Result<(), ConnectError>
     where T: IntoConnectParams
 {

tests/test.rs

@@ -644,7 +644,7 @@ fn test_cancel_query() {
     let _t = thread::spawn(move || {
         thread::sleep(Duration::from_millis(500));
         assert!(postgres::cancel_query("postgres://postgres@localhost", SslMode::None,
-                                       cancel_data).is_ok());
+                                       &cancel_data).is_ok());
     });
 
     match conn.execute("SELECT pg_sleep(10)", &[]) {