Unchecked value should always be a string if not null

[jho406]

This makes it consistent with hidden input values in HTML. Which is how unchecked values are used.

Relevant snippet In check_box.rb,

module FormProps
  module Inputs
    class CheckBox < Base
      def initialize(object_name, method_name, template_object, checked_value, unchecked_value, options)
        @checked_value = checked_value
        @unchecked_value = unchecked_value
        super(object_name, method_name, template_object, options)
      end