Problem with BertTokenizer using additional_special_tokens
# 🐛 Bug

## Information

Model I am using: Bert (bert-base-uncased)

Language I am using the model on: English

The problem arises when using:
* [ ] the official example scripts: (give details below)

The tasks I am working on is:
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. import transformers
2. from transformers import BertTokenizer
3. specify additional_special_tokens as ["[E11]", "[E12]", "[E21]", "[E22]"]
4. instantiate tokenizer as `tokenizer = BertTokenizer.from_pretrained('bert-base-uncased', do_lower_case=True, additional_special_tokens=additional_special_tokens)`
5. tokenize test string with the special tokens `'[E11] Tom Thabane [E12] resigned in October last year to form the [E21] All Basotho Convention [E22] -LRB- ABC -RRB- , crossing the floor with 17 members of parliament , causing constitutional monarch King Letsie III to dissolve parliament and call the snap election .'` using `tokenizer.tokenize(test_string)`

<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     Important! Use code tags to correctly format your code. See https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks#syntax-highlighting
     Do not use screenshots, as they are hard to read and (more importantly) don't allow others to copy-and-paste your code.-->

## Expected behavior

I would expect that the tokenization would give me regular tokens from wordpiece and the special tokens would be kept intact as [E11], [E12], etc, but instead I get:

`['[', 'e', '##11', ']', 'tom', 'tha', '##bane', '[', 'e', '##12', ']', 'resigned', 'in', 'october', 'last', 'year', 'to', 'form', 'the', '[', 'e', '##21', ']', 'all', 'bas', '##otho', 'convention', '[', 'e', '##22', ']', '-', 'l', '##rb', '-', 'abc', '-', 'rr', '##b', '-', ',', 'crossing', 'the', 'floor', 'with', '17', 'members', 'of', 'parliament', ',', 'causing', 'constitutional', 'monarch', 'king', 'lets', '##ie', 'iii', 'to', 'dissolve', 'parliament', 'and', 'call', 'the', 'snap', 'election', '.']`

I'm trying to run a training from [https://github.com/mickeystroller/R-BERT](https://github.com/mickeystroller/R-BERT), and reported this to the author, but he seems to get the proper results, even though we're both using transformers 2.8.0:

His results:

![image](https://user-images.githubusercontent.com/12713359/81397632-1b293600-90fe-11ea-8d91-a7af43aeabae.png)

My results:

![image](https://user-images.githubusercontent.com/12713359/81397653-241a0780-90fe-11ea-9ba6-1d86c2435dfd.png)

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.8.0
- Platform: Linux-4.15.0-99-generic-x86_64-with-debian-stretch-sid
- Python version: 3.7.5
- PyTorch version (GPU?): 1.3.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes, GTX 1070 with CUDA 10.1.243
- Using distributed or parallel set-up in script?: No

Here's the output from R-BERT's author as well:

- `transformers` version: 2.8.0
- Platform: Linux-4.15.0-72-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.4
- PyTorch version (GPU?): 1.4.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

The author from R-BERT seems to be using `tokenizers version 0.5.2`, and mine is 0.7.0. I tried downgrading mine to 0.5.2 to see if I would get the same results he did, but this doesn't seem to work because it's not compatible with transformers 2.8.0, as can be seen below. I have no Idea how he was able to use both together:

```
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-514ca6d60059> in <module>
----> 1 import transformers
      2 from transformers import BertTokenizer

/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/transformers/__init__.py in <module>
     53 from .configuration_xlm_roberta import XLM_ROBERTA_PRETRAINED_CONFIG_ARCHIVE_MAP, XLMRobertaConfig
     54 from .configuration_xlnet import XLNET_PRETRAINED_CONFIG_ARCHIVE_MAP, XLNetConfig
---> 55 from .data import (
     56     DataProcessor,
     57     InputExample,

/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/transformers/data/__init__.py in <module>
      4 
      5 from .metrics import is_sklearn_available
----> 6 from .processors import (
      7     DataProcessor,
      8     InputExample,

/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/transformers/data/processors/__init__.py in <module>
      3 # module, but to preserve other warnings. So, don't check this module at all.
      4 
----> 5 from .glue import glue_convert_examples_to_features, glue_output_modes, glue_processors, glue_tasks_num_labels
      6 from .squad import SquadExample, SquadFeatures, SquadV1Processor, SquadV2Processor, squad_convert_examples_to_features
      7 from .utils import DataProcessor, InputExample, InputFeatures, SingleSentenceClassificationProcessor

/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/transformers/data/processors/glue.py in <module>
     21 
     22 from ...file_utils import is_tf_available
---> 23 from ...tokenization_utils import PreTrainedTokenizer
     24 from .utils import DataProcessor, InputExample, InputFeatures
     25 

/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/transformers/tokenization_utils.py in <module>
     27 from typing import List, Optional, Sequence, Tuple, Union
     28 
---> 29 from tokenizers import AddedToken, Encoding
     30 from tokenizers.decoders import Decoder
     31 from tokenizers.implementations import BaseTokenizer

ImportError: cannot import name 'AddedToken' from 'tokenizers' (/media/discoD/anaconda3/envs/fast-bert/lib/python3.7/site-packages/tokenizers/__init__.py)
```

I've also confronted this issue. 
In my case, with `transformers v2.8.0` it correctly splits special token. 
But from `v2.9.0`, it splits the special tokens like `'[', 'e', '##11', ']'`. It's quite weird...<|||||>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.
