Correct authorID in Changeset on modification

This partly fixes the problem however the original authors UI isn't updated to be the same as the other authors in the room.  Ping pong may be required..

The bug this fixes is where if you drag a users content from point A to point B the author representation isn't changed.  Same with cut/paste.  This is bad as documented here: http://mclear.co.uk/2013/04/06/getting-author-ownership-right-in-a-collaborative-editor/

We still have a long way to go but this also stops users being booted if they try to merge a change that isn't there own

Ready to test.