binding esc key ot  combobulate-proffer-map failed.

  I bind two keys in `combobulate-proffer-map`, but esc failed, and C-e succeed, why ?

```elisp
(keymap-set combobulate-proffer-map "ESC" 'cancel)
(keymap-set combobulate-proffer-map "C-e" 'cancel)
```
When I press ESC, it keep going to "commit...", and C-e, "cancel..."