Fixed duplicate keys

Fix for tis bug:
```
$ cat test.db 
{"key":"k","val":"v1"}
{"key":"k"}
{"key":"k","val":"v2"}
```
```javascript
$ cat app.js 
var db = require('dirty')('test.db').on('load', function() {
  db.forEach(console.log);
});
```
```
$ node app.js 
k v2
k v2
```