It is invalid to set bounce in URDF

1. I am using Gazebo8+ubuntu16.04. 2. I set the bounce parameter in URDF, however, when I spawn model in Gazebo, the gazebo shows bounce parameter in the corresponding link is `restitution_coefficient="0.0" threshold="100,000.00"`,so it is invaild. 3. If I set bounce parameter in the sdf file, then it is valid.