As in CsvEncoder.php file, we have only the below constants defined, why are ...

As in CsvEncoder.php file, we have only the below constants defined, why are we using this constant NO_HEADERS_KEY.

I have removed this as this constant is not present in CsvEncoder.php file, only the below are defined.
const FORMAT = 'csv';
const DELIMITER_KEY = 'csv_delimiter';
const ENCLOSURE_KEY = 'csv_enclosure';
const ESCAPE_CHAR_KEY = 'csv_escape_char';
const KEY_SEPARATOR_KEY = 'csv_key_separator';
const HEADERS_KEY = 'csv_headers';

Please review this patch.