Allow other plugins to set a default for multi value checkboxes

[jaapjansma]

Before

If another plugin such as Caldera CiviCRM Form Processor would set a default value for a multi checkbox, the default could only be one single value. Thus allowing checking only one of the checkboxes and not multiple at the same time.

After

If another plugin such as (Caldera CiviCRM Form Processor)[https://github.com/CiviMRF/cf-civicrm-formprocessor/] would set a default value for a multi checkbox, the default could be an array and thus multiple checkboxes would be checked.

Why is this needed?

We use calder forms for an update your profile allowing a user to update their first name, last name, email address and the list of subscribed email newsletters. The current information is retrieved from CiviCRM. For the newsletter we created a checkbox field and options for each newsletter. A user could be subscribed to multiple newsletters at the same time. (You can see the form at https://staging.civicoop.org/testpagina-inschrijven-nieuwsbrie)

[jaapjansma]

I am not sure why the tests are failing