Fix compiling libserialize tests

They've got new warnings turned on so they need more `#![allow]`

Author: alexcrichton

src/libserialize/tests/json.rs

@@ -1,3 +1,5 @@
+#![allow(rustc::default_hash_types)]
+
 extern crate serialize as rustc_serialize;
 
 use rustc_serialize::{Encodable, Decodable};

src/libserialize/tests/opaque.rs

@@ -1,3 +1,5 @@
+#![allow(rustc::default_hash_types)]
+
 extern crate serialize as rustc_serialize;
 
 use rustc_serialize::{Encodable, Decodable};