# File lib/rmail/header.rb, line 570
    def params(field_name, default = nil)
      if params = params_quoted(field_name)
        params.each { |name, value|
          params[name] = value ? Utils.unquote(value) : nil
        }
      else
        if block_given?
          yield field_name
        else
          default
        end
      end
    end