Can focus menu dividers with the keyboard

[klmcdorm]

jsFiddle:​https://jsfiddle.net/bmu3k2w3/2/
If a menu has a divider as the first or last item, it can be focused using the keyboard. Reproduction steps:

1. Click on the input box and press Tab to focus on the menu.
2. Move focus to the last menu item by pressing the down arrow key.
3. Try to move focus back to the first menu item by pressing down again.
4. Focus moves to the divider before the first item instead. In the menu's focus event, ui.item is the divider.

This also happens when moving up from the first item, or when using Page Up or Page Down.
Tested on Windows 7 with Chrome 56.0.2924.87, Firefox 52.0.1, and Internet Explorer 11.0.9600.18537.

[Scott González]

​https://github.com/jquery/jquery-ui/pull/1804

[Scott González]

In a3e953b:

Menu: Don't focus dividers when wrapping via keyboard navigation

Fixes #15157
Closes gh-1804