Iterating over a vision dataset doesn't decode the images
## Describe the bug

If I load `mnist` and I iterate over the dataset, the images are not decoded, and the dictionary with the bytes is returned.

## Steps to reproduce the bug
```python
from datasets import load_dataset
import PIL

mnist = load_dataset("mnist", split="train")

first_image = mnist[0]["image"]
assert isinstance(first_image, PIL.PngImagePlugin.PngImageFile)  # passes
first_image = next(iter(mnist))["image"]
assert isinstance(first_image, PIL.PngImagePlugin.PngImageFile)  # fails
```

## Expected results

The image should be decoded, as a PIL Image

## Actual results

We get a dictionary
```
{'bytes': b'\x89PNG\r\n\x1a\n\x00..., 'path': None}
```

## Environment info

- `datasets` version: 1.17.1.dev0
- Platform: Darwin-20.6.0-x86_64-i386-64bit
- Python version: 3.7.2
- PyArrow version: 6.0.0

The bug also exists in 1.17.0

## Investigation

I think the issue is that decoding is disabled in `__iter__`:

https://github.com/huggingface/datasets/blob/dfe5b73387c5e27de6a16b0caeb39d3b9ded66d6/src/datasets/arrow_dataset.py#L1651-L1661

Do you remember why it was disabled in the first place @albertvillanova ?
Also cc @mariosasko @NielsRogge 
