Silently enabling sessions

[larryfox]

I just tracked down an issue we we're having with sessions which was caused because this gem silently enables sessions at the Sinatra level. Our product spans a couple Sinatra and plain Rack apps so we use Rack::Session::Cookie as a normal middleware, which causes conflicts with the "Sinatra way".

I'm not sure if it's feasible but I like the approach rack_csrf takes that checks for sessions or raises an error.

[jsmestad]

@larryfox happy to review and merge in any suggested changes if you can get to that.

[jsmestad]

Just updated the gem and saw your issue. If you can detail a bit of how you'd see this working, I'd try and pull in the change.