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.