Add validation of boolean type fields

[mjordan]

Currently, boolean fields like published (which is named status internally to Drupal) are not validated to ensure they only contain either a 1 or a 0. If any other values appear in the JSON pushed up to Drupal, the request fails with a 422 status and a primitive type error message.

Workbench already validates other field types so this new validation can use that established pattern.