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 17b32b764acd815400d9eb903aaec6dcebd60ac7
Merge branch 'justincc-master'