add BOM for UTF8

faster_csv.rb

class FasterCSV
  def <<(row)
    # make sure headers have been assigned
    if header_row? and [Array, String].include? @use_headers.class
      parse_headers  # won't read data for Array or String
      self << @headers if @write_headers
    end

    # Handle FasterCSV::Row objects and Hashes
    row = case row
          when self.class::Row then row.fields
          when Hash            then @headers.map { |header| row[header] }
          else                      row
          end

    @headers =  row if header_row?
    @lineno  += 1

    row[0] = "\357\273\277" + row[0] if @lineno == 1

    @io << row.map(&@quote).join(@col_sep) + @row_sep  # quote and separate

    self  # for chaining
  end
end