BertTokenizer.save_vocabulary() doesn't work as docstring described
## 🐛 Bug
https://github.com/huggingface/pytorch-transformers/blob/1b35d05d4b3c121a9740544aa6f884f1039780b1/pytorch_transformers/tokenization_bert.py#L169-L174

## Expected behavior

It's obvious that when `vocab_path` is not a directory, the `vocab_file` is not defined.
I believe replacing all `vocab_path` with `vocab_file` solves this issue, vice versa.