Check for parser instance instead of class name.

When creating a CSV parser plugin to override the default FeedsCSVParse, the feeds_tamper checks for the class name (hardcoing the class "FeedsCSVParser") instead of checking if the parser is of instance of FeedsCSVParser. Getting the class name wrong creates a wrong value for the $is_csv variable. Using "instanceof" instead of get_class() can avoid this issue.