BERT Tokenization problem when the input string has a "." in the string, like floating number
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): BERT

Language I am using the model on (English, Chinese ...): English

The problem arises when using: Tokenizer
* [ ] the official example scripts: (give details below) N/A
* [ ] my own modified scripts: (give details below) N/A

The tasks I am working on is: Any
* [ ] an official GLUE/SQUaD task: (give the name) N/A
* [ ] my own task or dataset: (give details below) N/A

## To reproduce

Steps to reproduce the behavior:

1.  Load any BERT tokenizer
2. Tokenize something with a "." in between
3. Decode these ids, you will find it mismatch

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```
x = tokenizer.encode('AN.C', add_special_tokens=False)
z = tokenizer.decode(x)
```
It prints:
```
AN. C
```

## Expected behavior
```
AN.C
```
<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version:
- Platform: Ubuntu
- Python version: 3.6
- PyTorch version (GPU?): GPU
- Tensorflow version (GPU?): GPU
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
