Checkout gateway purchase request issues

[simonworkhouse]

It appears that the resulting response from an Omnipay\Stripe\Message\Checkout\PurchaseRequest request always returns true from the isSuccessful method; whereas I would have expected for it to respond with a specific redirect response and to always fall back to false for calls to isSuccessful.

Additionally, the completePurchase method hasn't been implemented and that would at least be one that I'd expect to see for a payment method like this. Perhaps the intent was to utilise the capture method instead?

There's also the issue of line items not being pre-filled from the ItemBag and other required parameters not having default values set.

I could submit a pull request for these updates (if I get time), but I'm just wanting to confirm whether or not this is actually the intended behaviour.

So is this the intended behaviour?