If an agent is sitting, then do send the rotation in the agent update instead of zeroing it to resolve mouselook camera problems

Addresses http://opensimulator.org/mantis/view.php?id=6892
Thanks to tglion for this spot.
This resolves a recent regression from 17b32b764a

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -1640,6 +1640,8 @@ namespace OpenSim.Region.Framework.Scenes
                 ControllingClient.SendAgentTerseUpdate(this);
 
             PhysicsActor actor = PhysicsActor;
+
+            // This will be the case if the agent is sitting on the groudn or on an object.
             if (actor == null)
             {
                 SendControlsToScripts(flagsForScripts);