FIX: Use the correct variable as the key into $record

It was using $fieldName, which is the CSV field name, not the database
field name. This prevents duplicate detection from working. It now
properly uses $SQL_fieldName.

Update CsvBulkLoaderTest to remove keys that are nonexistent in the CSV
test data. Having them causes the test to fail with an undefined-index
error. This did not previously fail because of the bug in CsvBulkLoader
that this patch fixes. This partially reverts c4eac53.
Merge pull request #1819 from oddnoc/csvbulkloader-findexisting-3.1

Use the correct variable as the key into $record