Fix up tests

Author: kyeah

tests/modules/bson.rs

@@ -17,9 +17,9 @@ fn to_json() {
 
     let second = obj.get("second").unwrap();
     assert!(second.is_string());
-    assert_eq!(second.as_string().unwrap(), "foo");
+    assert_eq!(second.as_str().unwrap(), "foo");
 
     let alphanumeric = obj.get("alphanumeric").unwrap();
     assert!(alphanumeric.is_string());
-    assert_eq!(alphanumeric.as_string().unwrap(), "bar");
+    assert_eq!(alphanumeric.as_str().unwrap(), "bar");
 }

tests/modules/oid.rs

@@ -1,6 +1,5 @@
 use bson::oid::ObjectId;
 use rustc_serialize::hex::ToHex;
-use rustc_serialize::json;
 
 #[test]
 fn deserialize() {