Triggering the MenuButton using the keyboard cause the top menu item to be triggered.

[mynameisstephen]

1. Add a alert or console.log to vjs.MenuItem.prototype.onClick
2. Using the keyboard tab to a Menu Button such as Captions hit enter.
3. The top menu item is triggered.

[heff]

Thanks for finding this! I don't know yet if this is the same issue but I discovered the keyboard events are bubbling unexpectedly and submitted #1452. I'm hoping fixing that one can also fix this issue, though I'm about to dig into your PR too.