Space token cannot be add when is_split_into_words = True
for example, 
```python
>>> tokenizer = BertTokenizer.from_pretrained('bert-base-chinese')
>>> tokenizer.add_tokens(' ')
1
```

```python
>>> tokenizer.encode('你好 世界', add_special_tokens=False)
[872, 1962, 21128, 686, 4518]
>>> tokenizer.encode(['你','好',' ', '世', '界'], is_split_into_words=True, add_special_tokens=False)
 [872, 1962, 686, 4518]
```
Obviously, the blank token is ignored. But if you change it to another token like ‘[balabala]’, it works.
So what is the proper way to do this?


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