class SimpleForm::Inputs::BooleanInput

def build_check_box(unchecked_value='0')

in Rails > 3.2.1, and is backported in SimpleForm AV helpers.
which won't generate the hidden checkbox. This is the default functionality
reuse the method for nested boolean style, but with no unchecked value,
Build a checkbox tag using default unchecked value. This allows us to 
def build_check_box(unchecked_value='0')
  @builder.check_box(attribute_name, input_html_options, '1', unchecked_value)
end