Key sequence specs appear to be case-sensitive in test helpers, but not in `on-key` modifier

I was defining an `{{on-key "Shift+Enter" this.someCallback}}` in my app code, which worked fine. But then when I went to make an acceptance test and added `keyDown("Shift+Enter")`, this would not fire the callback.

I tried changing it to `keyDown("shift+enter")` in the test and this still didn't work.

Finally, I made both specify in all lowercase, `{{on-key "Shift+Enter" this.comeCallback}}` in the template and `keyDown("shift+enter")` in the test and now it does trigger the callback.

I know this now and can work around it, but it doesn't seem like intended behaviour to me. It would be ideal if both parts behaved the same, case-insensitive way, but I'd be just as happy if all lowercase were required in both contexts and this were documented/enforced with a warning.