Perform self-collision checks with padded robot.

Self-collision checking is currently done with an unpadded robot. My expectation is that if a robot has a padding specification, it should not only be used for environment collision checks, but also for self-collision checks.

I've tested this patch on my end and it works, but I don't know to which extent it might be affecting code that actually expects self-collision checks to be unpadded.

See also @k-okada's comment on https://github.com/ros-planning/moveit_ros/issues/341#issuecomment-32701710.