Make sure we always create UTF-8 XML

WorldPay really doesn't like entities -- but it's totally fine
with proper UTF-8 strings.

Change-Id: I24c71710cbda047e0abef8c31fe964b19ffbc3cb

Conflicts:

	tests/DonationInterfaceTestCase.php
	tests/includes/Responses/worldpay/AuthorizePaymentForFraud.testresponse