ArrowInvalid: Could not convert <PIL.Image.Image image mode=RGB when adding image to Dataset
## Describe the bug

When adding a Pillow image to an existing Dataset on the hub, `add_item` fails due to the Pillow image not being automatically converted into the Image feature.

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

dataset = load_dataset("hf-internal-testing/example-documents")

# load any random Pillow image
image = Image.open("/content/cord_example.png").convert("RGB")

new_image = {'image': image}
dataset['test'] = dataset['test'].add_item(new_image)
```

## Expected results
The image should be automatically casted to the Image feature when using `add_item`. For now, this can be fixed by using `encode_example`:

```
import datasets

feature = datasets.Image(decode=False)
new_image = {'image': feature.encode_example(image)}
dataset['test'] = dataset['test'].add_item(new_image)
```

## Actual results

```
ArrowInvalid: Could not convert <PIL.Image.Image image mode=RGB size=576x864 at 0x7F7CCC4589D0> with type Image: did not recognize Python value type when inferring an Arrow data type
```
