fix(rustup): rustc 1.0.0-nightly (123a754cb 2015-03-24)

* fix `extern crate` declaration for rustc-serialize
* enable `into_cow` feature
* replace as_slice() calls by as_ref and enable `convert` feature
* use `core` feature in doc tests

src/header/common/accept_language.rs

@@ -54,7 +54,7 @@ mod tests {
     #[test]
     fn test_parse_header() {
         let a: AcceptLanguage = Header::parse_header(
-            [b"en-us;q=1.0, en;q=0.5, fr".to_vec()].as_slice()).unwrap();
+            [b"en-us;q=1.0, en;q=0.5, fr".to_vec()].as_ref()).unwrap();
         let b = AcceptLanguage(vec![
             qitem(Language{primary: "en".to_string(), sub: Some("us".to_string())}),
             QualityItem::new(Language{primary: "en".to_string(), sub: None},