Incorrect URDF to SDF conversion of gravity and self_collide tags

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/71)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

The original report had attachments: [test_urdf.sdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/71/attachments/test_urdf.sdf), [test_urdf.urdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/71/attachments/test_urdf.urdf), [test_urdf.xacro](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/71/attachments/test_urdf.xacro)

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Here is the [source](http://answers.gazebosim.org/question/7074/urdf-to-sdf-conversion-using-gzsdf/#), and the specific part of the question:

**gravity, selfCollide and self_collide (gazebo tag for links)**

Adding one of the tags above for a link, an additional tag is added to the resulting sdf file, i.e. there are then two gravity tags for the same link in the sdf file where the first is always set to true (1) and the second sdf tag holds the value set in the urdf. I assume that when gazebo parses the sdf file, it always finds the first tag using their value, thus always neglecting what has been set in the urdf.

and

**kp, kd (gazebo tag for joint)**

Not converted into sdf. Why?