Fix record.clear()

This was more of a traceur bug. Funny thing: `_empty` was leaking onto `Immutable.Record` after the first `clear` on an instance.

```js
var MyRec = Immutable.Record({a:1});
var myRec = new MyRec();
myRec.clear();
Immutable.Record._empty._map === myRec._map
// => true
```