#34 Fixed 'Cannot pass token and PayerID to completePurchase'

See https://github.com/thephpleague/omnipay-paypal/issues/34
What are we testing there is that GET arguments are actually used for getting paypal token and payerid. Basically we are testing that correct paypal request is issued.

We cannot use ExpressPurchaseSuccess.txt to check if correct paypal request is issued because it contains paypal response, not request.

I'm not sure how should I address this comment :(