How to download exported csv file in WordPress

I work on the plugin that creates menu in wp-admin/ side and shows the table with some data. I need to generate CSV ( it’s going correct ) and give user to donwload it automatically. I know, that I have to add the headers like these

header('Content-type: application/csv');
header('Content-Disposition: attachment; filename="' . $csv_file_name . '"');

But unfortunately it does not work for wordpress admin-side.
Again – the generating of CSV is going correct, but it displays just a text of csv file below the table, but don’t give the file

Here is full code

if ( isset( $_REQUEST['export_csv'] ) ) {
        global $wpdb;
        $csv_source_array = $wpdb->get_results ( $wpdb->prepare( " SELECT name, email, time, text FROM {$table_name} " ), ARRAY_N );
        $csv_file_name = 'nba.rally.'.date(Ymd).'.csv';
        $csv_header_array = array( "Name", "Email", "Date", "Message" );        

            if (isset($csv_source_array)) {

                header('Content-type: application/csv');
                header('Content-Disposition: attachment; filename="' . $csv_file_name . '"');

            ob_start();
                $f = fopen('php://output', 'w') or show_error("Can't open php://output");
                $n = 0;


                    if (isset($csv_header_array)) {
                        if ( !fputcsv($f, $csv_header_array, ';'))
                        {
                            echo "Can't write line $n: $line";
                        }
                    }

                foreach ($csv_source_array as $line)
                {
                    $n++;

                    if ( !fputcsv($f, $line, ';'))
                    {
                        echo "Can't write line $n: $line";
                    }
                }
                fclose($f) or show_error("Can't close php://output");
                $csvStr = ob_get_contents();
                ob_end_clean();

                echo $csvStr;
            }


        }   

Thanks for an advance for any answers.