Issues with RobertaTokenizer and unicode characters
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...):
Roberta

Language I am using the model on (English, Chinese ...):
English

## To reproduce

Steps to reproduce the behavior:

1. Insert a jumble of some weird unicode characters into the vocabulary file. These unicode characters must exist on their own somewhere else in the vocab. (Note: I didn't actually do this manual vocab editing when I encountered the bug. Since I'm using a custom vocabulary I'm simply detailing the steps to reproduce the behavior)
``` 
# vocab.txt
...
ĠOHÃ¢
...
```
(Let's say this was inserted on line 1001)
(Don't forget to account for this extra word with a change in vocab size in your config file)

2. Load an instance of `RobertaTokenizer` with this vocab file.
3. Call 
```python3
input_ids = tokenizer.encode_plus('OHÃ¢', 
                                  add_special_tokens=True,
                                  add_prefix_space=True,
                                  max_length=max_seq_length,
                                  return_token_type_ids=True)['input_ids']
print(input_ids)
```


## Expected behavior

The special start-of-word character `Ġ` will be added by the tokenizer, so the input ids we'd expect would be `[0, 1000, 2]` (given that `0` and `2` correspond to your start/end tokens, and `1000` corresponds to the index where we inserted the unicode jumble `ĠOHÃ¢`). Instead, I'm getting some seemingly unrelated input ids corresponding to other weird unicode characters not included in the inserted unicode jumble. 

## Environment info     
- `transformers` version: 2.8
- Platform: Linux and MacOS
- Python version: 3.7
- PyTorch version (GPU?): torch 1.4.
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
