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/shared/encoding.rs

@@ -31,7 +31,7 @@ impl fmt::Display for Encoding {
             Deflate => "deflate",
             Compress => "compress",
             Identity => "identity",
-            EncodingExt(ref s) => s.as_slice()
+            EncodingExt(ref s) => s.as_ref()
         })
     }
 }