do_lower_case not saved/loaded correctly for Tokenizers
## Environment info
- `transformers` version: 3.4.0
- Platform: Linux-5.4.0-52-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.5.1 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@mfuntowicz

## Information

The `do_lower_case` property of BertTokenizer is not correctly restored after saving / loading.

## To reproduce

```python
from transformers import BertTokenizer

tokenizer = BertTokenizer.from_pretrained("bert-base-cased")
print(tokenizer.do_lower_case)

tokenizer.save_pretrained("debug_tokenizer")
tokenizer_loaded = BertTokenizer.from_pretrained("debug_tokenizer")
print(tokenizer_loaded.do_lower_case)

``` 
returns
```
False
True
``` 
## Expected behavior
Same object attributes after saving / loading 
Oh! I'll take a look, thanks for the report @tholor <|||||>Thanks for the fast fix @thomwolf ! Very much appreciated!