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/method.rs

@@ -102,7 +102,7 @@ impl fmt::Display for Method {
             Trace => "TRACE",
             Connect => "CONNECT",
             Patch => "PATCH",
-            Extension(ref s) => s.as_slice()
+            Extension(ref s) => s.as_ref()
         })
     }
 }