Never split some texts.
I have noticed bert tokenize texts by two steps:
1. punctuation: split text to tokens
2. wordpiece: split token to word_pieces

Some texts such as `"[UNK]"` are supposed to be left as they are. However, they become `["[", "UNK", "]"]` or something like this.

This PR is to solve the above problem.
Please have a look. :)<|||||>Looks good indeed, thanks @WrRan!