Quickpay v7 has an "acquirers" field in requests

The original implementation of protocol v7 called this field "acquirer" (without the "s"), which matched old and incorrect documentation provided in the past by Quickpay. We learned via Quickpay support (through debug of a merchant issue) that the field in the request is called "acquirers", representing a list of acquirers to attempt, ordered by preference (i.e. `acquirers=nets,teller`).  The response will contain a field called "acquirer" which conveys which acquirer was actually used in the transaction.

The correct documentation can now be found on the Quickpay website:
http://doc.quickpay.net/api/messagetypes.html#index1h2
http://doc.quickpay.net/api/messagetypes.html#index2h2

Before this change, attempting to set the old `:acquirer` option would result in an MD5check hash mismatch, since the ActiveMerchant code would use the `acquirer` field in the calculation, but Quickpay would use `acquirers`.

After this change, both ActiveMerchant and Quickpay agree on the MD5check hash when the `:acquirers` option is set, and the `:acquirers` option has the intended effect of setting the preferred acquirers for the transaction. (The latter can be negative-tested by setting the value for acquirers to an invalid acquirer value and observing the error response: "Error in field: acquirers")

Despite the change to the option key (`acquirer` to `acquirers`), I feel this is a safe, backwards compatible change since attempting to set the old option could never result in a successful transaction - the MD5check hash mismatch error would always occur.

Note: I asked Quickpay to update all of their v7 documentation to reflect the correct field name.  At this time it appears they have done so.