BertTokenizer loses unicode character
## Environment info

- `transformers` version: 3.0.2
- Platform: Linux-4.15.0-109-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.4.0 (True)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No


### Who can help
 @LysandreJik 


## Information

The tokenizer seems to lose a specific unicode character on tokenization.  From the sentence

`General Saprang Kalayanamitr ( Thai : <unk> ่ ง กัลยาณมิตร ;`

from line 2557 of the Wiki02 training dataset, there is a little dot after and above `<unk>`
however the tokenizer produces 

` ['general', 'sap', '##rang', 'kala', '##yana', '##mit', '##r', '(', 'thai', ':', '<', 'un', '##k', '>', 'ง', '[UNK]', ';']`


## To reproduce

`t = BertTokenizer.from_pretrained('bert-base-uncased')`
`o = t.tokenize('General Saprang Kalayanamitr ( Thai : <unk> ่ ง กัลยาณมิตร ;')`
 `o`
`['general', 'sap', '##rang', 'kala', '##yana', '##mit', '##r', '(', 'thai', ':', '<', 'un', '##k', '>', 'ง', '[UNK]', ';']`

## Expected behavior

A subword should be given for the ' ่ ' token.  Otherwise there should be an option to be given a warning for removed characters.

I'm having a similar problem since upgrading to 4.0 where certain unicode characters are being "eaten" even though I set 'use_fast' = False.

## Environment info

- `transformers` version: 4.0.0
- Platform: macOS-10.16-x86_64-i386-64bit
- Python version: 3.8.6
- PyTorch version (GPU?): 1.7.0 (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

BertTokenizer

The problem comes from certain unicode character being not turned correctly into UNK characters by the tokenizers when they did in earlier versions.

## To reproduce

```
import transformers
import torch
from transformers import BertTokenizer

print(torch.__version__)
print(transformers.__version__)

# THERE IS A UNICODE character between the , and '' (specifically \U+200D\U+200D\U+200D\U+200D)
sentence = ": ءُپَاعَر سِيْا , ‍‍‍‍ '' Upal"

tokenizer = BertTokenizer.from_pretrained(
            "bert-base-cased",
            do_lower_case=False,
            use_fast=False
        )

print(tokenizer.tokenize(sentence))

#  output 4.0.0

[':', '[UNK]', '[UNK]', ',', "'", "'", 'Up', '##al']

# output from 3.0.1

[':', '[UNK]', '[UNK]', ',', '[UNK]', "'", "'", 'Up', '##al'
```<|||||>This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

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