uniqBy only works when iteratee returns a primitive value

I was expecting the following:

```
_.uniqBy([{ 'x': { 'y': 1 } }, { 'x': { 'y': 2 } }, { 'x': { 'y': 1 } }], 'x');
```

to return:
```
[{ 'x': { 'y': 1 } }, { 'x': { 'y': 2 } } }]
```

but it returns:
```
[{ 'x': { 'y': 1 } }, { 'x': { 'y': 2 } }, { 'x': { 'y': 1 } }]
```

Is this a bug or just an undocumented restriction?