save_pretrained on CamembertTokenizer
## 🐛 Bug

This is probably something you already know, but `save_pretrained` for `CamembertTokenizer` seems not working at the moment.

Here is the error message I get:

```
---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-38-52393f32f8a0> in <module>
      1 results = {}
      2 if args.do_eval and args.local_rank in [-1, 0]:
----> 3     tokenizer = tokenizer_class.from_pretrained(args.output_dir, do_lower_case=args.do_lower_case)
      4     checkpoints = [args.output_dir]
      5     if args.eval_all_checkpoints:

/mnt/azmnt/code/Users/adm/transformers/transformers/tokenization_utils.py in from_pretrained(cls, *inputs, **kwargs)
    281 
    282         """
--> 283         return cls._from_pretrained(*inputs, **kwargs)
    284 
    285 

/mnt/azmnt/code/Users/adm/transformers/transformers/tokenization_utils.py in _from_pretrained(cls, pretrained_model_name_or_path, *init_inputs, **kwargs)
    410 
    411         # Instantiate tokenizer.
--> 412         tokenizer = cls(*init_inputs, **init_kwargs)
    413 
    414         # Save inputs and kwargs for saving and re-loading with ``save_pretrained``

/mnt/azmnt/code/Users/adm/transformers/transformers/tokenization_camembert.py in __init__(self, vocab_file, bos_token, eos_token, sep_token, cls_token, unk_token, pad_token, mask_token, additional_special_tokens, **kwargs)
     55         self.max_len_sentences_pair = self.max_len - 4  # take into account special tokens
     56         self.sp_model = spm.SentencePieceProcessor()
---> 57         self.sp_model.Load(str(vocab_file))
     58         # HACK: These tokens were added by fairseq but don't seem to be actually used when duplicated in the actual
     59         # sentencepiece vocabulary (this is the case for <s> and </s>

/anaconda/envs/azureml_py36/lib/python3.6/site-packages/sentencepiece.py in Load(self, filename)
    116 
    117     def Load(self, filename):
--> 118         return _sentencepiece.SentencePieceProcessor_Load(self, filename)
    119 
    120     def LoadOrDie(self, filename):

OSError: Not found: "None": No such file or directory Error #2

```

Best
This should be fixed in #1860 which should be merged shortly