RobertaTokenizer fails to do_lower_case, different behavior between version 2 and 3
## Environment info
     
- `transformers` version: 3.4.0 / 2.8.0
- Platform: linux
- Python version: 3.8

### Who can help
@mfuntowicz 

## Information

Tokenizer I am using: RobertaTokenizer
The tokenizer do not lower case the text even if I explicitly set do_lower_case=True. The behavior is different between version 2.8.0 and 3.4.0

## To reproduce

Steps to reproduce the behavior:

```python
from transformers import RobertaTokenizer
tokenizer = RobertaTokenizer.from_pretrained("roberta-base", do_lower_case=True)
print(tokenizer.tokenize("Huggingface"))
```

## Expected behavior

Version 3.4.0 prints: ['Hug', 'ging', 'face']
Version 2.8.0 prints: ['h', 'ug', 'ging', 'face']

Could you please also try with the most recent transformers release and report what happens?<|||||>Version 4.0.1 prints: ['Hug', 'ging', 'face'] <|||||>Explicitly setting the attribute 'do_lower_case' to True solves the problem.

```python
from transformers import RobertaTokenizer
tokenizer = RobertaTokenizer.from_pretrained("roberta-base", do_lower_case=True)
tokenizer.do_lower_case = True
print(tokenizer.tokenize("Huggingface"))
```

<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>If we use the AutoTokenizer library, this still does not work. 

```
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("roberta-base", do_lower_case=True)
tokenizer.do_lower_case = True
print(tokenizer.tokenize("Huggingface"))
```