Disable Sinatra sessions for Rails apps

If you enable sessions in a Sinatra app, and then mount it in a Rails 4 app,
Rack::Session will blow up because ActionDispatch::Request::Session does not
implement #each.
Don't output the csrf token if the csrf_token is nil

If sessions fail, and the csrf_token is nil, we don't want to send an
authenticity_token param with requests otherwise Rack::Protection will deny
the request (because nil != "")
Merge pull request #62 from toolmantim/fix-rails-sessions

Fix Rails session problems