Do not generate hidden check box field when using nested boolean style

It is considered invalid markup in HTML5. This will only work in
Rails > 3.2.1 (not released at this time). If you really care that much
about this, please use Rails 3-2-stable branch for now, that contains a
backported fix from master.

Backport commit: rails/rails@2e5ec3b
More info in #215

CHANGELOG.md

@@ -27,6 +27,9 @@
   * Add possibility to give a block to `collection_radio` and `collection_check_boxes`,
     to generate custom label and input structure. It is used internally with the :nested
     option for `:boolean_style`, and is useful to allow some more customization if required.
+  * Do not generate hidden check box field when using nested boolean style, as it is considered
+    invalid markup in HTML5. This will only work in Rails > 3.2.1 (not released at this time).
+    More info in [#215](https://github.com/plataformatec/simple_form/issues/215)
 
 ### deprecation
   * Deprecate the `translate` configuration in favor of `translate_labels`

lib/simple_form/inputs/boolean_input.rb

@@ -2,21 +2,28 @@ module SimpleForm
   module Inputs
     class BooleanInput < Base
       def input
-        @builder.check_box(attribute_name, input_html_options)
+        build_check_box
       end
 
       def label_input
         if options[:label] == false
           input
         elsif nested_boolean_style?
-          @builder.label(label_target, label_html_options) { input + label_text }
+          @builder.label(label_target, label_html_options) { build_check_box(nil) + label_text }
         else
           input + label
         end
       end
 
       private
 
+      # Build a checkbox tag using default unchecked value. This allows us to
+      # reuse the method for nested boolean style, but with nil unchecked value,
+      # which won't generate the hidden checkbox (only in Rails > 3.2.1).
+      def build_check_box(unchecked_value='0')
+        @builder.check_box(attribute_name, input_html_options, '1', unchecked_value)
+      end
+
       # Booleans are not required by default because in most of the cases
       # it makes no sense marking them as required. The only exception is
       # Terms of Use usually presented at most sites sign up screen.

lib/simple_form/inputs/collection_check_boxes_input.rb

@@ -10,7 +10,7 @@ def has_required?
       end
 
       def nested_boolean_style_item_tag(value, html_options)
-        @builder.check_box(attribute_name, html_options, value)
+        @builder.check_box(attribute_name, html_options, value, nil)
       end
     end
   end