`bert-base-uncased` tokenizer broke around special tokens in v2.2.1
In `v2.2.1`, the `bert-base-uncased` tokenizer changed in a way that's probably not intentional:

```
Python 3.7.5 (default, Oct 25 2019, 10:52:18)
[Clang 4.0.1 (tags/RELEASE_401/final)] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from transformers.tokenization_auto import AutoTokenizer
To use data.metrics please install scikit-learn. See https://scikit-learn.org/stable/index.html
Neither PyTorch nor TensorFlow >= 2.0 have been found.Models won't be available and only tokenizers, configurationand file/data utilities can be used.
>>> t = AutoTokenizer.from_pretrained("bert-base-uncased"); t.encode_plus(text='A, [MASK] AllenNLP sentence.')
{
    'input_ids': [101, 1037, 1010, 1031, 7308, 1033, 5297, 20554, 2361, 6251, 1012, 102],
    'token_type_ids': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
}
```

In `v2.2.0`:
```
Python 3.7.5 (default, Oct 25 2019, 10:52:18)
[Clang 4.0.1 (tags/RELEASE_401/final)] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from transformers.tokenization_auto import AutoTokenizer
To use data.metrics please install scikit-learn. See https://scikit-learn.org/stable/index.html
Neither PyTorch nor TensorFlow >= 2.0 have been found.Models won't be available and only tokenizers, configurationand file/data utilities can be used.
>>> t = AutoTokenizer.from_pretrained("bert-base-uncased"); t.encode_plus(text='A, [MASK] AllenNLP sentence.')
{
    'special_tokens_mask': [1, 0, 0, 0, 0, 0, 0, 0, 0, 1],
    'input_ids': [101, 1037, 1010, 103, 5297, 20554, 2361, 6251, 1012, 102],
    'token_type_ids': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
}
```
(indented the results for clarity)

The key difference is that in `v2.2.0`, it recognizes the `[MASK]` token as a special token and gives it token id `103`. In `v2.2.1`, this no longer happens. The behavior of `bert-base-cased` has not changed, so I don't think this is an intentional change.


`git bisect` says the commit introducing this problem is 7246d3c2f93c4461f3ec8ada7a26a002d8f196ea.<|||||>Any way you could run the same test on `master`? It might have been fixed since.<|||||>I did. It was not fixed in master.

It only affects the [MASK] token.

On Tue, Dec 10, 2019, 16:25 Julien Chaumond <notifications@github.com>
wrote:

> Any way you could run the same test on master? It might have been fixed
> since.
>
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<|||||>I screwed up. It is fixed in `master` after all.<|||||>Good to hear! We'll push a new release soon, cc @LysandreJik 