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/connection.rs

@@ -46,7 +46,7 @@ impl fmt::Display for ConnectionOption {
         write!(fmt, "{}", match *self {
             KeepAlive => "keep-alive",
             Close => "close",
-            ConnectionHeader(UniCase(ref s)) => s.as_slice()
+            ConnectionHeader(UniCase(ref s)) => s.as_ref()
         })
     }
 }