Fix typo
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205?src=pr&el=h1) Report
> Merging [#1205](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/0b52642d379bed155e8aa4f4088588bfd8ceaa88?src=pr&el=desc) will **increase** coverage by `0.44%`.
> The diff coverage is `95.65%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1205      +/-   ##
==========================================
+ Coverage   80.83%   81.27%   +0.44%     
==========================================
  Files          46       46              
  Lines        7878     7877       -1     
==========================================
+ Hits         6368     6402      +34     
+ Misses       1510     1475      -35
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [...h\_transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvdG9rZW5pemF0aW9uX3Rlc3RzX2NvbW1vbnMucHk=) | `100% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1205/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3V0aWxzLnB5) | `89.37% <91.66%> (+8.89%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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<|||||>Thanks a lot for that!
Took the occasion to add regression tests and clean up a bit the base class.