Menu item attributes are registered incorrectly

[kaidesu]

Describe the issue

• After creating a new navigation menu and assigning fields, the added fields are not correctly registered when using the menu() helper method.
• Link target also needs to be defined even if New Window was not set. Otherwise, some items have a target property and others don't. This will cause confusion and issues.

Expected behavior

To simply access fields directly off each item:

{{ $item->foo }}
{{ $item->bar }}

Version and browser

Version: Beta 5

Bug report checklist

• I have filled out as much information as I can
• I have detailed how the issue can be reproduced
• I have searched for existing issues and to the best of my knowledge this is not a duplicate