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/upgrade.rs

@@ -37,7 +37,7 @@ impl fmt::Display for Protocol {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{}", match *self {
             WebSocket => "websocket",
-            ProtocolExt(ref s) => s.as_slice()
+            ProtocolExt(ref s) => s.as_ref()
         })
     }
 }