Cannot set checkboxes to be checked by default using def

## To Reproduce

Step by step instructions to reproduce the behavior:

> 1. Create a widget. In index.js create a field with the type checkboxes
> 2. Enter def value. which is the default value for the field(Values must be from the defined choices' values) Or enter def: true
> 3. Checkbox should now be checked when adding that field to the page, but it isn´t

## Expected behavior
> Checkbox should be checked when def value is set

Reproduce the issue by defining a checkbox field with a def value in index.js, using one of the defined choice values or true. Trace the checkbox field's default-value handling and verify that adding the field to a page renders it checked when def is set.

Written by the indexing model from the issue text.