Avoid error on redirection when "rack.session" is nil

When my server make a redirection without "rack.session" rack-google-analytics raise ```NoMethodError: undefined method `[]=' for nil:NilClass``` on https://github.com/leehambley/rack-google-analytics/blob/fb76f71fa3becfad1fa08138b301409a07bb0517/lib/rack/google-analytics.rb#L35

This PR fix it.