FIX: CSVBulkLoader duplicate check can now check more than one field

Instead of CsvBulkLoader->findExistingRecord out right failing (i.e. no duplicate found) when the duplicate check field is empty, it will now continue on to check other duplicateCheck fields. This way you don't get duplicate records being created, because the **first** found duplicate check field is empty.

Added extra testing data to CSVBulkLoaderTest so that it fails.

This issue was first raised here: http://open.silverstripe.org/ticket/6255 (2 years ago lol)
It was also submitted here: https://github.com/silverstripe/sapphire/pull/10 (I think it was mixed up with some other changes however)

@chillu