Add support to action-cache json requests wrapped in a callback (jsonp)

Given the following controller:

class PostsController < ApplicationController
  caches_action :index

  def index
    @posts = Post.all
    render :json => @posts, :callback => params[:callback]
  end
end

It's not possible to successfully cache json requests with callbacks (jsonp) because of the nature of how callbacks work.

This patch modifies the caches_action around filter by resetting the callback before rendering the fragment if it's a json request.

NOTE: This implementation saves the fragment with callback and reset it on read. Another implementation could be to remove the callback on save, and add (if necessary) on read.