unk_token not set when loading TransformerXLTokenizer.from_pretrained() from a save_pretrained() 
Using `TransfomerXLTokenizer.from_pretrained()` when loading from `.save_pretrained()` generated files.

Python complains about missing <unknown> token: 

```
line 175, in _build_from_file
    raise ValueError("No <unkown> token in vocabulary")
ValueError: No <unkown> token in vocabulary
```
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.
