add_tokens() leading to wrong behavior
Hey folks, I am using GPT2Tokenizer and tried add_tokens() below. But it gives unintended behavior and this is because of #612 in tokenization_utils.py
                sub_text = sub_text.strip()
As you see, dollars is split because of space removal. Could someone look into it please.

```
t1 = GPT2Tokenizer.from_pretrained('gpt2')
t1.tokenize('my value in dignums dollars')
```
Output - ['my', 'Ġvalue', 'Ġin', 'Ġdign', 'ums', '**Ġdollars**']

```
t2 = GPT2Tokenizer.from_pretrained('gpt2')
t2.add_tokens(['dignums'])
t2.tokenize('my value in dignums dollars')
```
Output - ['my', 'Ġvalue', 'Ġin', 'dignums', **'d', 'oll', 'ars'**]
I guess that's why add_prefix_space=True would be needed here.