This field is very similar to the Boolean Widget except that this field allows other positive values
besides 1. Since unselected checkboxes do not return a parameter, fields with Checkbox type will always
be set to the 'input_without_param' default if they do not appear in the form.
widget
checkbox
checkbox_value
In order to create the HTML for a checkbox, there must be a 'value="xx"'. This value is specified with
the 'checkbox_value' attribute, which defaults to 1.
input_without_param
If the checkbox is not checked, it will be set to the value of this attribute (the unchecked value).
Default = 0. Because unchecked checkboxes do not return anything in the HTTP parameters, the absence of a
checkbox key in the parameters hash forces this field to this value. This means that Checkbox fields,
unlike other fields, will not be ignored if there is no input. If a particular checkbox should not be
processed for a particular form, you must set 'inactive' to 1 instead.
Note that a checkbox is only 'checked' when the 'checkbox_value' is provided. The 'value' for a non-
checked checkbox is only really useful for creating form values such as are stored in a database.