def use_ssl=(flag)
      flag = flag ? true : false
      if started? and @use_ssl != flag
        raise IOError, "use_ssl value changed, but session already started"
      end
      @use_ssl = flag
    end
def use_ssl=(flag)
      flag = flag ? true : false
      if started? and @use_ssl != flag
        raise IOError, "use_ssl value changed, but session already started"
      end
      @use_ssl = flag
    end
Turn on/off SSL.
 This flag must be set before starting session.
 If you change use_ssl value after session started,
 a Net::HTTP object raises IOError.