Fix lowercase bug in wordpiece tokenizer

[abuelnasr0]

This PR fixes a bug in wordpiece tokenizer and it addresses this comment #1395 (comment)

[mattdangerw]

Thanks! Looks good to me. Let's just add a comment to explain.

[abuelnasr0]

@mattdangerw last two commits are not necessary. First, I changed it to two stars "**" instead of "६", because "**" is impossible to be in the string so there is no possibillity to build a wrong mask. but I forgot that even if the mask was build wrong, There will be no effect, because tf_text.case_fold_utf8(text) have no effect on "६".

anyways I have a question about this two lines: https://github.com/keras-team/keras-nlp/blob/9ac333581fa55216ecd11e0fc566e12daef1cf82/keras_nlp/tokenizers/word_piece_tokenizer.py#L152-L153

what is the purpose of them?
at the first glance they were not necessary for me. and I removed them, and all tests passed.
We are splitting in cjx_regex and whitespace down, so no need to add white space before and after every cjx char? WDYT?

[mattdangerw]

what is the purpose of them?
at the first glance they were not necessary for me. and I removed them, and all tests passed.
We are splitting in cjx_regex and whitespace down, so no need to add white space before and after every cjx char? WDYT?

We might not! Here's context #318 and https://github.com/google-research/bert/blob/master/tokenization.py#L251-L262

I'll pull in this fix for now, as we will probably be cutting a release tomorrow, but we can follow up with any additional changes here.