"input" event should not fire when a key does not change field value

The ["input" event spec](https://html.spec.whatwg.org/multipage/indices.html#event-input-input) states that the event is fired when the value of an input field changes.

This implies that the event should not be fired when a keypress does not actually change the value of the input and that is the case as far as I can tell in Chrome and Firefox.

In syn however the "input" event is fired when [any key](https://github.com/bitovi/syn/blob/5151fc2864e519bce1499ebb649495de5bc0a52b/src/key.js#L853) is pressed.

Start in src/key.js around line 853 and reproduce the behavior for keys that leave the field value unchanged. Compare the synthetic event behavior with the input event specification and browser behavior. Done means the input event is not fired when a key does not change the field value.

Written by the indexing model from the issue text.