Standardize on using the oxford/serial comma with lists of 3 or more.
This seemed the far more prevalent convention in the file but there
were 5 instances where it was not used.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@830 edb9625f-4e0d-4859-8d74-9fd3b1da38cb