Preserve `list` type of `additional_special_tokens` in `special_token_map`
Author: SaulLu
Pull request: yes

# What does this PR do?

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Fixes #12598

As shown in the issue, the value associated with the `additional_special_tokens` key in the `special_tokens_map` attribute of a tokenizer was of type string while its content is rather a list. This PR proposes a fix to keep the list type and adds tests.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. @LysandreJik and @sgugger, I would love to have a second look if there was ever a reason I missed that required the list to become a string.
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