Quickpay: Adds currency for v7 store operations

While working with Quickpay in production mode, it was discovered that "amount" and "currency" are required values for store (subscribe) operations in the v7 API.  Without it, subscribe operations complain with "Error in request parameters". This is confirmed by their documentation, which currently states that both "amount" and "currency" are required:

http://doc.quickpay.net/api/messagetypes.html#index2h2

This commit makes the change such that only the subscribe parameters for v7 (and up) are changed.  New unit tests were added to prove the correct parameters are now being sent, and all existing remote tests were run and are passing.