Fix Rails session problems

This fixes problems introduced in #57 which added CSRF protection

An isolated example which causes `NoMethodError: undefined method `each' for #<ActionDispatch::Request::Session:0x7fdf5aacc8a8 not yet loaded>` in `rack-1.5.2/lib/rack/session/abstract/id.rb in stringify_keys line: 158`:

```ruby
require "action_controller/railtie"
require "logger"
require "sinatra/base"

class SinatraApp < Sinatra::Base
  enable :sessions # Disabling this will make everything AOK

  get "/" do
    "Hello"
  end
end

class RailsApp < Rails::Application
  config.logger = Rails.logger = Logger.new(STDOUT)
  config.secret_key_base = "foo"

  routes.draw do
    get "/" => SinatraApp
  end
end

run RailsApp
```

Fixes #41. Fixes #59.

Still no tests in this gem :(