Add a hack to work around a bug that CheckBoxField may not even try to get a property value, because it has a default value 0 in the field definition itself. The same hack was performed against Form.py but this has been disabled mistakenly for a while, because monkey-patched methods were overridden in ProxyField.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@38432 20353a03-c40f-0410-a6d1-a30d3c3de9de