Empty multiple_checkbox is not saved

[frafra]

Hi! :)
In certain cases, changes to a multiple_checkbox are not saved.

How to reproduce:

1. Create a schema with a multiple_checkbox
2. Create a dataset and check some boxes
3. Edit the dataset and uncheck all the boxes

The last action has no effect. This happens because the /edit/ endpoint calls package_patch instead of package_update.

ckanext-scheming/ckanext/scheming/views.py

Line 184 in 8646a9d

complete_data = get_action('package_patch')(None, data_dict)

Since package_patch has to be used, since multiple start_form_page can be specified, a reasonable fix would be to have the browser send the entire form, including an empty list for each of the empty multiple_checkbox.