tft.scale_to_z_score_per_key should support non-string keys like tft.vocabulary does

[cyc]

As it says in the title, tft.vocabulary allows you to pass in a feature of type tf.int[8|16|32|64] in addition to tf.string. It would be good if tft.scale_to_z_score_per_key could support this as well. Of course, I know I can just convert my int-valued keys to strings before calling tft.scale_to_z_score_per_key, but it would be easier if I didn't have to.