    # File lib/rmail/header.rb, line 596
        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