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/charset.rs

@@ -105,7 +105,7 @@ impl Display for Charset {
 impl FromStr for Charset {
     type Err = ();
     fn from_str(s: &str) -> Result<Charset, ()> {
-        Ok(match s.to_ascii_uppercase().as_slice() {
+        Ok(match s.to_ascii_uppercase().as_ref() {
             "US-ASCII" => Us_Ascii,
             "ISO-8859-1" => Iso_8859_1,
             "ISO-8859-2" => Iso_8859_2,

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()
         })
     }
 }