addBefore failed

```
import OrderedMap from 'orderedmap';

function run() {
    let om = OrderedMap.from({});

    om = om.addToStart('a', 'aa');
    om = om.addToEnd('d', 'dd');
    om = om.addBefore('c', 'b', 'bb');
    om = om.addBefore('d', 'c', 'cc');

    om.forEach((k, v) => console.log(k, v));
}
```

expect
```console.log
a aa
b bb
c cc
d dd
```

actual
```console.log
a aa
c cc
d dd
b bb
```