Move the hidden :id field logic to where it belongs to

When dealing with nested forms, Rails automatically generates a hidden
field with the id value of the current object being generated by
fields_for. This logic was inside the method that's available from the
template object, but we just need it when really dealing with nested
attributes, so moving the code to here makes more sense.

actionpack/lib/action_view/helpers/form_helper.rb

@@ -705,9 +705,7 @@ def apply_form_for_options!(record, object, options) #:nodoc:
       # to prevent fields_for from rendering it automatically.
       def fields_for(record_name, record_object = nil, options = {}, &block)
         builder = instantiate_builder(record_name, record_object, options)
-        output = capture(builder, &block)
-        output.concat builder.hidden_field(:id) if output && options[:hidden_field_id] && !builder.emitted_hidden_id?
-        output
+        capture(builder, &block)
       end
 
       # Returns a label tag tailored for labelling an input field for a specified attribute (identified by +method+) on an object
@@ -1824,9 +1822,14 @@ def fields_for_nested_model(name, object, options, block)
           object = convert_to_model(object)
 
           parent_include_id = self.options.fetch(:include_id, true)
-          include_id = options.fetch(:include_id, parent_include_id)
-          options[:hidden_field_id] = object.persisted? && include_id
-          @template.fields_for(name, object, options, &block)
+          include_id        = options.fetch(:include_id, parent_include_id)
+          hidden_field_id   = object.persisted? && include_id
+
+          @template.fields_for(name, object, options) do |f|
+            output = @template.capture(f, &block)
+            output.concat f.hidden_field(:id) if output && hidden_field_id && !f.emitted_hidden_id?
+            output
+          end
         end
 
         def nested_child_index(name)