GPT2Tokenizer doesn't include BOS or EOS token
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): GPT-2

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

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

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

## To reproduce

Steps to reproduce the behavior:

Script:
```
from transformers import GPT2Tokenizer

tokenizer = GPT2Tokenizer.from_pretrained("gpt2")
encoded_dict = tokenizer.encode_plus(text="Hello I am Moin", add_special_tokens=True, \
    max_length=512, truncation_strategy="longest_first", pad_to_max_length=False, \
    return_tensors=None, return_token_type_ids=True, return_attention_mask=True, \
    return_overflowing_tokens=False, return_special_tokens_mask=False)

print(tokenizer.bos_token_id)
print(encoded_dict['input_ids'])
```

You should see that the `input_ids` do not include the `bos_token_id`. Shouldn't `encode_plus` be doing this?

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## Expected behavior
The <|endoftext|> token would appear, since I included to `add_special_tokens`.

## Environment info
<!-- You can run the command `python 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: Linux-4.15.0-54-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.2
- PyTorch version (GPU?): 1.3.1 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

Accidental double post -- closing this in favour of #3311 