load_dataset ignoring features
Hi ! Thanks for reporting. I opened a PR to fix this issue: #2201 Nice question which helped me a lot! I have wasted a lot of time to the `DatasetDict` creation from a csv file. Hope the document of this module add some simple examples. Hi :) We're indeed working on tutorials that we will add to the docs !
First of all, I'm sorry if it is a repeated issue or the changes are already in master, I searched and I didn't find anything. 

I'm using datasets 1.5.0

![image](https://user-images.githubusercontent.com/37592763/113114369-8f376580-920b-11eb-900d-94365b59f04b.png)

As you can see, when I load the dataset, the ClassLabels are ignored, I have to cast the dataset in order to make it work.

Code to reproduce:

```python
import datasets
data_location = "/data/prueba_multiclase"
features = datasets.Features(
        {"texto": datasets.Value("string"), "label": datasets.features.ClassLabel(names=["false", "true"])}
    )
dataset = datasets.load_dataset(
        "csv", data_files=data_location, delimiter="\t", features=features
    )
```

Dataset I used:


[prueba_multiclase.zip](https://github.com/huggingface/datasets/files/6235022/prueba_multiclase.zip) (it has to be unzipped)


Thank you! ❤️ 
