Supports custom merchantCode in paymentService
Supports custom merchantCode in paymentService

Depending on the customer’s configuration of the merchant facilities, WorldPay could require the `merchantCode` in the top level XML tag `paymentService` to be a different `merchant_code` that’s not the `login`. This change adds support for this. Note: the `merchant_code` needs to be passed to the initialiser of the `@gateway` so it’s added to the `@options` and not just on the `options` parameter of a `purchase` method, say …

```
$ bundle exec ruby -Itest test/unit/gateways/worldpay_test.rb
Loaded suite test/unit/gateways/worldpay_test
Started
.....................................

Finished in 0.196618 seconds.
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37 tests, 210 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
188.18 tests/s, 1068.06 assertions/s
```

and remote tests

```
$ bundle exec ruby -Itest test/remote/gateways/remote_worldpay_test.rb
Loaded suite test/remote/gateways/remote_worldpay_test
Started
Finished in 72.670442 seconds.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
23 tests, 66 assertions, 6 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
73.913% passed
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.32 tests/s, 0.91 assertions/s
```

note: of the 6 failures, all return `"Order not ready”` and the one from `test_successful_credit_on_cft_gateway` as we don’t have proper credentials for the CFT gateway.

waysact/active_merchant,waysact/active_merchant