Rails does not generate a hidden field for checkboxes if
`unchecked_value` is false. The nested boolean creates its own hidden
field and ignores this rule from rails. That means that the hidden
field is generated for a falsey value when using the nested style, but
not when using the inline style.
This commit fixes this problem by making the nested style behave like
the inline style, adhering to the standard set by rails.
Resolves: #1320
dfcd041b22