Checkbox attribute fields are not set on the model when opening the shortcode UI

[goldenapples]

From wp-shortcake/shortcake-bakery#165

Currently in our shortcode model, it's expected that a checkbox field should store a boolean value.

However, when we format a shortcode string, we don't treat boolean fields any differently than other field types, and serialize them as quoted values, ie attribute="true".

When opening the shortcode UI a second time for the shortcode, this field is then read as a string, rather than a boolean, and isn't set properly on the shortcode model (callbacks on the field will report its changed value as "true", rather than true, for example).

Related issues: #550, #359