AssertionError in official example 
## 🐛 Bug

<!-- Important information -->

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

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

The problem arise when using:
* [x] the official example scripts: (give details)
* [ ] my own modified scripts: (give details)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details)

## To Reproduce

Steps to reproduce the behavior:

in the [official example](https://huggingface.co/transformers/quickstart.html) 
the tokenizer result raise an assertionError

```
text = "[CLS] Who was Jim Henson ? [SEP] Jim Henson was a puppeteer [SEP]"
tokenized_text = tokenizer.tokenize(text)

# Mask a token that we will try to predict back with `BertForMaskedLM`
masked_index = 8
tokenized_text[masked_index] = '[MASK]'
assert tokenized_text == ['[CLS]', 'who', 'was', 'jim', 'henson', '?', '[SEP]', 'jim', '[MASK]', 'was', 'a', 'puppet', '##eer', '[SEP]']

```
gives 

```
AssertionError                            Traceback (most recent call last)
<ipython-input-1-6533b2fb8252> in <module>
     16 masked_index = 8
     17 tokenized_text[masked_index] = '[MASK]'
---> 18 assert tokenized_text == ['[CLS]', 'who', 'was', 'jim', 'henson', '?', '[SEP]', 'jim', '[MASK]', 'was', 'a', 'puppet', '##eer', '[SEP]']
     19 
     20 # Convert token to vocabulary indices

AssertionError: 

```
And when i print out the tokenized_text, i find the sepcial tokens had been tokenized in a wrong way.
This may caused by the lower operation to the speicial tokens

```
print(tokenized_text)
['[', 'cl', '##s', ']', 'who', 'was', 'jim', 'henson', '[MASK]', '[', 'sep', ']', 'jim', 'henson', 'was', 'a', 'puppet', '##eer', '[', 'sep', ']']
```

## Expected behavior

no exception.

## Environment

* OS: Ubuntu 19.04 / Centos ?
* Python version: 3.7.5 / 3.6.4
* PyTorch version: 1.2.0
* PyTorch Transformers version (or branch): 2.2.1
* Using GPU ? maybe
* Distributed of parallel setup ? No
* Any other relevant information:

The official example is ok in Transformers version 2.1.1, after i update my Transformers it goes wrong

## Additional context

Dpulicated to #2052 and closed it .

