Small fixes
Author: thomwolf
Pull request: yes

Fix #908 and #901

Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=h1) Report
> Merging [#911](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/c054b5ee64df1a180417c5e87816879c93f54e17?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `90.9%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
+ Coverage   79.03%   79.04%   +0.01%     
==========================================
  Files          34       34              
  Lines        6234     6242       +8     
==========================================
+ Hits         4927     4934       +7     
- Misses       1307     1308       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3V0aWxzLnB5) | `86.56% <90.9%> (+0.03%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=footer). Last update [c054b5e...7b6e474](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=h1) Report
> Merging [#911](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/c054b5ee64df1a180417c5e87816879c93f54e17?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `90.9%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
+ Coverage   79.03%   79.04%   +0.01%     
==========================================
  Files          34       34              
  Lines        6234     6242       +8     
==========================================
+ Hits         4927     4934       +7     
- Misses       1307     1308       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3V0aWxzLnB5) | `86.56% <90.9%> (+0.03%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=footer). Last update [c054b5e...7b6e474](https://codecov.io/gh/huggingface/pytorch-transformers/pull/911?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).