Cannot use triggerKeyEvent using key of lowercase letters

Currently there is a check to not allow lowercase `key`s. I presume this is for a quick sanity check for non-printable keys like `Enter` etc. As a workaround, I need to use the `keyCode` instead.
```js
triggerKeyEvent(SELECTOR, 'keydown', 'A'); // uppercase 'A'
triggerKeyEvent(SELECTOR, 'keydown', 65); // lowercase 'a'
```

Is it possible to extend the method argument to allow `key` lowercase letters ?