Testing that batch_encode_plus is the same as encode_plus
Spoiler alert: it wasn't.

closes #2960 
closes #2658 
closes #2654 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2973?src=pr&el=h1) Report
> Merging [#2973](https://codecov.io/gh/huggingface/transformers/pull/2973?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c36416e53c29da8b6193f4a36d7b024c5f513495?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `92.85%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2973/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2973?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2973      +/-   ##
==========================================
+ Coverage   77.14%   77.17%   +0.03%     
==========================================
  Files          98       98              
  Lines       16006    16020      +14     
==========================================
+ Hits        12348    12364      +16     
+ Misses       3658     3656       -2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2973?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2973/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.83% <100%> (+0.11%)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2973/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `91.06% <92.3%> (+0.45%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2973?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/transformers/pull/2973?src=pr&el=footer). Last update [c36416e...9e3275a](https://codecov.io/gh/huggingface/transformers/pull/2973?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
