Fix circular argument reference for ruby 2.2.0

.../ruby/gems/2.2.0/bundler/gems/rack-simple_csrf-f86a150086ea/lib/rack/simple_csrf.rb:94:
warning: circular argument reference - session
.../ruby/gems/2.2.0/bundler/gems/rack-simple_csrf-f86a150086ea/lib/rack/simple_csrf.rb:99:
warning: circular argument reference - session