Decode now calls private property instead of public method
Removes the warning raised when the decode method is called.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?src=pr&el=h1) Report
> Merging [#1087](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/e00b4ff1de0591d5093407b16e665e5c86028f04?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `33.33%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #1087      +/-   ##
=========================================
- Coverage   79.61%   79.6%   -0.02%     
=========================================
  Files          42      42              
  Lines        6898    6898              
=========================================
- Hits         5492    5491       -1     
- Misses       1406    1407       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3V0aWxzLnB5) | `85.9% <33.33%> (-0.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?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/1087?src=pr&el=footer). Last update [e00b4ff...2ba1a14](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1087?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Yes! Thanks @LysandreJik LGTM