Chunk padding

Hi,

The chunk function doesn't behave quite as described in the docs which state that a non-array padding argument is repeated to fill out the final chunk if it's incomplete.

Also trying to pad with a falsey value was indistinguishable from chunking without a pad argument - I used arguments.length to fix this.

There's also a small typo in the documentation for the chunk function:

```javascript
_.chunk([0,1,2,3,4], 3, '_');
//=> , [[0,1,2],[3,'_','_']]
```

should read:
```javascript
_.chunk([0,1,2,3], 3, '_');
//=> , [[0,1,2],[3,'_','_']]
```

(i.e. the '4' from the input array should be removed to match the output). I didn't update the docs to comply with CONTRIBUTING.md.

Thanks
Dave