dataset_view() does not return all files

In version 1.5.6, the `api.dataset_view()` function does not return the names of any files that are not in the local root directory.

Example:

```python
>>> import kaggle #v1.5.6
>>> dataset_name = '4quant/depth-generation-lightfield-imaging'
>>> api = kaggle.KaggleApi()
>>> api.authenticate()
>>> dataset = api.dataset_view(dataset_name)
>>> dataset.files
[depth_training_data.npz]
```

An inspection of [4quant/depth-generation-lightfield-imaging on kaggle.com](https://www.kaggle.com/4quant/depth-generation-lightfield-imaging) shows that there are many files contained in subdirectories (e.g. `3d_scenes/3d_scenes/img00032.txt`) that are not returned by the API call.