SagePay: Truncate more fields

SagePay is somewhat particular about field lengths; transactions are
declined if those lengths are exceeded.  Now we truncate more of the
fields to match their docs:

http://www.sagepay.co.uk/file/1166/download-document/DIRECTProtocolandIntegrationGuidelinesV3%200%20(1).pdf

Closes #1293.