def build_header
      header = Header.new
      header.push_data("callerid", @caller_id)
      header.push_data("service", @service_name)
      header.push_data("md5sum", @service_type.md5sum)
      header.push_data("type", @service_type.type)
      if @persistent
	header.push_data("persistent", '1')
      end
      header
    end
def build_header
      header = Header.new
      header.push_data("callerid", @caller_id)
      header.push_data("service", @service_name)
      header.push_data("md5sum", @service_type.md5sum)
      header.push_data("type", @service_type.type)
      if @persistent
	header.push_data("persistent", '1')
      end
      header
    end
@param [String] host host name
 @param [Integer] port port number
 @param [String] caller_id caller_id of this node
 @param [String] service_name name of service
 @param [Class] service_type class of this service
 @param [Boolean] persistent use persistent connection or not

 build henader message for service client.
 It contains callerid, service, md5sum, type, persistent.
 @return [Header] header