Keep even the first of the special tokens intact while lowercasing
This fixes #2220. The order of `all_special_tokens` is random, and the first one of these will get broken by the lowercasing. There are 5 special tokens, so you have a 1 in 5 chance of hitting the problem.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2232?src=pr&el=h1) Report
> Merging [#2232](https://codecov.io/gh/huggingface/transformers/pull/2232?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a5a06a851e1da79138e53978aa079a093f243dde?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2232/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2232?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2232   +/-   ##
=======================================
  Coverage   81.43%   81.43%           
=======================================
  Files         122      122           
  Lines       18338    18338           
=======================================
  Hits        14933    14933           
  Misses       3405     3405
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2232?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2232/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `90.49% <100%> (ø)` | :arrow_up: |

------

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<|||||>> This fixes #2220. The order of `all_special_tokens` is random, and the first one of these will get broken by the lowercasing. There are 5 special tokens, so you have a 1 in 5 chance of hitting the problem.

You're right! Good job @dirkgr :-)<|||||>Great, thanks a lot for that @dirkgr. I've added a test in fb393ad9945f66b081f88b81b90a2974d81e9601 to make sure this doesn't happen again.