Add special Response for Checkout Session

[vojtasvoboda]

Stripe Checkout Purchase request works fine, but it creates Stripe Session Response which is not compatible with standard Response:

• for Checkout there is always redirect, no matter on 3DS availability
• redirect URL is in $data['url'] parameter not in $data['redirect']['url']

So I've created a new Checkout\Response. With this small fix, isRedirect() returns true and I'm able to redirect to the Checkout gateway.

[adnane-ka]

I know this might be a bit late but yeah! This pull request should be definitely considered. I had to create my own handler when returned offsite, which clearly messed up my payment logic flow across other multiple omnipay gateways.