I don't believe Mongrel has the capacity to handle SSL requests; you'll need Apache or Nginx for that. You could deal with the annoyance of running Apache on your development boxes, but you'd need to set up appropriate SSL certificates on each machine, which is simply not worth the hassle.
However, even if you disable SSL in your development environment you can leave it on in your test environment and write functional tests for the behavior you want around SSL. For instance, you could test your FooController with something like this:
describe "#show" do it "should redirect to SSL" do
request.env['HTTPS'] = nil get :show, :id => @foo.to_param response.should redirect_to(request.url.sub(/^http/, "https"))
I'd suggest you set up a demo or staging environment to manually test that SSL is working properly. Something like Heroku is great for this.
what is the reason to for testing ssl in development? i personally think this is best left to server config.