Fix lever-arm handling in the revolute joint.

src/dynamics/joint/revolute_joint.rs

@@ -72,7 +72,7 @@ impl RevoluteJoint {
             motor_max_impulse: Real::MAX,
             motor_impulse: 0.0,
             prev_axis1: *local_axis1,
-            motor_model: SpringModel::VelocityBased,
+            motor_model: SpringModel::default(),
             motor_last_angle: 0.0,
         }
     }

src/dynamics/joint/spring_model.rs

@@ -21,6 +21,12 @@ pub enum SpringModel {
     ForceBased,
 }
 
+impl Default for SpringModel {
+    fn default() -> Self {
+        SpringModel::VelocityBased
+    }
+}
+
 impl SpringModel {
     /// Combines the coefficients used for solving the spring equation.
     ///