(#112,#113) fix service persist without keyward

rospy : rospy.ServiceProxy(name, service_class, persistent=True)
roscpp : createClient (const std::string &service_name, bool persistent=false, const M_string &header_values=M_string())

I'm not sure if we should add keyward for persist or not in roseus case, but this implementation support

ros::service-call (name value &optional (persist nil))
Merge pull request #117 from k-okada/fix_service_persist

(#112,#113) fix service persist without keyward