make save_pretrained work with added tokens

right now it's dumping the *decoder* when it should be dumping the *encoder*. this fixes that.

pytorch_transformers/tokenization_utils.py

@@ -266,7 +266,7 @@ class PreTrainedTokenizer(object):
 
         with open(added_tokens_file, 'w', encoding='utf-8') as f:
             if self.added_tokens_encoder:
-                out_str = json.dumps(self.added_tokens_decoder, ensure_ascii=False)
+                out_str = json.dumps(self.added_tokens_encoder, ensure_ascii=False)
             else:
                 out_str = u"{}"
             f.write(out_str)