Use _.identity in _.sortBy if iterator is not specified

For me, it was a suprise, that `_.sortBy` without iterator is no-op.

```js
_.sortBy([3, 2, 1]); // returns [3, 2, 1];
```

Thiis PR makes `_.sortBy` use `_.identity` as default iterator, same as eg `_.groupBy` so:

```js
_.sortBy([3, 2, 1]); // returns [1, 2, 3]
```