Add custom attributes when serializing

This patch allows to add additional attributes when serializing via to_json or to_xml.

Specific use-case I have for it is when servicing ajax creation requests. As response, I want to return the created object information, but I would like to also provide information on how to render it (basically html snippet) so that js client can use it to update dom in a consistent manner. So I render the show template for it and mix into the rest of json data for the model:

  class FoosController < ApplicationController
    def create
      @foo = Foo.new params[:foo]
      if @foo.save
        respond_to do |f|
          f.html { redirect_to @foo }
          f.js { render :json => @foo.to_json(:merge => { :html => render_to_string(:action => 'show') }) }
        end
      else
        respond_to do |f|
          f.html { render :action => 'new' }
          f.js { render :json => @foo.to_json(:methods => :errors) }
        end
      end
    end
  end

The patch touches following files:

• active_record/serialization.rb to implement support of the :merge option
• test/cases/json_serialization_test.rb and test/cases/serialization_test.rb to reflect the new behavior
• active_record/serializers/json_serialization.rb to document the new feature.

Hope that sounds useful.

-- Dmitry

PS Current implementation allows the merged attributes to override the defaults. Also, it ignores excepted attributes (specified via :except option). The only reason they behave like that is because it made most sense to me.