Fix checkboxes checked state

[NicoHood]

This is a long standing issue (7 years) for checkbox default values when use: keys is not used. The default values are still specified in a named array defaultvalue => keyname but the code assumes a flat array. When setting a default value of false the code will fail. You can test this using:

my_field:
    type: checkboxes
    label: A couple of checkboxes
    default:
        option1: true
        option2: false
    options:
        option1: Option 1
        option2: Option 2

Edit: It seems there is more broken. I will work on that and edit.

[NicoHood]

I will close this for now, as the issue itself must be reevaluated: #471

[mahagr]

You cannot do that. If you're not using keys, default is something like this:

default:
  - option1
  - option2