FIX: If CSV column mapping maps to function, keep key value as key.

Fixes http://open.silverstripe.org/ticket/6473

When using CSVParser::$columnMapping to map columns to a callback action, it previously used the action name as the key value. This prevented users from defining multiple entries to the same callback. This patch retains those key values and simply runs the callback field name filter later on.
Code formatting / PHPDoc cleanup