Weird Behavior on XLNetTokenizer after new tokens added
## 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: 3.0.2
- Platform: ubuntu
- Python version: 3.7.7
- PyTorch version (GPU?): 1.5.1 (GPU)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@mfuntowicz
 
## Information
I use XLNetTokenizer (pretrained) and added a new token in it. After that, the output from ```tokenizer.tokenize``` looks weird.
(I am not sure if the problem comes from ```transformers``` or ```tokenizers```, but I post here anyway.)

## To reproduce
```
from transformers import XLNetTokenizer
tokenizer = XLNetTokenizer.from_pretrained('xlnet-base-cased')

test = 'This is so awesome!!@username!'
out = tokenizer.tokenize(test)

# without new tokens added, @username is broken down as expected
out
>> ['▁This', '▁is', '▁so', '▁awesome', '!!', '@', 'user', 'name', '!']
```

```
from tokenizers import AddedToken

# introduce new tokens (with white-space on right only)
new_tokens = AddedToken(
    '@username', single_word = True, 
    lstrip = False, rstrip = True
)
tokenizer.add_tokens(new_tokens)

out = tokenizer.tokenize(test)

# weird result about the white-space around new tokens
out
>> ['▁This', '▁is', '▁so', '▁awesome', '!!', '@username', '▁', '!']
```

In here, there are two places weird:
1. new tokens "@username" has a white-space on the left, so ""!!@username" should not be broken down into "!!", "@username" (I think it should be broken down into "!!", "@", "user", "name", "!")
2. I am a bit confused on why there is a white-space produced after "@username" token. (i.e. '@username', '▁', '!')

And oddly, when I encode and decode the sentence back, the white-space token after "@username" is not translated to actual whitespace. (Also note there is a white space added before "@username" which mean the new token is correctly identified to have a white-space on the left):
```
enc = tokenizer.encode(test, add_special_tokens = False)
dec = tokenizer.decode(enc)

# in encoding stage, the 2nd last token is whitespace
enc 
>> [122, 27, 102, 8729, 7675, 32000, 17, 136]
# in decoding stage, the white-space disappear

dec
>> This is so awesome!! @username!
```


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.
