Encoding special tokens
## 🐛 Bug

<!-- Important information -->
In version 2.2.1 encoding special tokens changed.

```python
from transformers import BertTokenizer

tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
tokenizer.decode(tokenizer.encode("[CLS] hello world [SEP]", add_special_tokens=False))
```
output: `'[ cls ] hello world [ sep ]'`

For version `transformers==2.2.0` the output is: `'[CLS] hello world [SEP]'`



I got the same issue for version 2.2.1. <|||||>I also meet this issue and you may check out the possible root cause from #2052. 

My workaround is backoff to 2.1.1 version.
<|||||>Should have been fixed with https://github.com/huggingface/transformers/pull/2051