Correct tokenization for special and added tokens
When a tokenizer is being loaded with `PreTrainedTokenizer._from_pretrained`, it should set `added_tokens` and `all_special_tokens` to `unique_added_tokens_encoder`. 
If we don't do it, it will corrupt the tokenization. 

Example:
```
import transformers
tokenizer = transformers.BertTokenizer.from_pretrained("bert-base-uncased")
tokenizer.tokenize("[CLS] token should not be splitted.")
# correct output
# ['[CLS]', 'token', 'should', 'not', 'be', 'split', '##ted', '.']
# incorrect output
# ['[', '[UNK]', ']', 'token', 'should', 'not', 'be', 'split', '##ted', '.']
```

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2312?src=pr&el=h1) Report
> Merging [#2312](https://codecov.io/gh/huggingface/transformers/pull/2312?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cea04a244351a7c5bce44e1cfc01abde0ceb60fd?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2312/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2312?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2312      +/-   ##
==========================================
+ Coverage   73.54%   73.54%   +<.01%     
==========================================
  Files          87       87              
  Lines       14789    14791       +2     
==========================================
+ Hits        10876    10878       +2     
  Misses       3913     3913
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2312?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2312/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `92.11% <100%> (+0.03%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2312/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `63.91% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2312?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/2312?src=pr&el=footer). Last update [cea04a2...b262577](https://codecov.io/gh/huggingface/transformers/pull/2312?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This is awesome, thanks a lot @vitaliyradchenko 