feature request: manually fire click event when keyDown key is enter or spacebar

per the w3c spec, when a focused element receives a keyDown event with the key `Enter` or ` `, browsers will construct a native `click` event on that focused element.

from https://w3c.github.io/uievents/#event-type-keydown:
> If the key is the Enter or key and the current focus is on a state-changing element, the default action MUST be to dispatch a click event, and a DOMActivate event if that event type is supported by the user agent (refer to § 3.5 Activation triggers and behavior for more details)

this issue is to request that ember-test-helpers follows the same pattern by modifying the `triggerKeyEvent` helper to also fire a click event in the cases described above.