Fix colladawriter overload, fixing skipwrite physics

[cielavenir]

https://github.com/rdiankov/openrave/pull/1262 had one regression, where if( listbodies.size() == 1 )RaveWriteColladaFile(listbodies.front(),filename,atts);. colladawriter has very special meaning in RaveWriteColladaFile(KinBodyPtr pbody) overload. So this must not be removed.

This caused SEGV, but now fixed.

openrave.py -i src/robots/pr2-beta-static.zae
env.Save('1.mujin.dae', Environment.SelectionOptions.Body, {'target': robot.GetName(), 'skipwrite': 'visual geometry readable sensors physics'})

controllercommon CI will not pass until this is merged. Sorry for the inconvenience.

/cc @felixvd @ntohge

[rdiankov]

thanks~