simplified CRS(), now accepting CRS(as.logical(NA)), reported by Robert Hijmans


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rspatial@1492 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
tightening CRS NA

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rspatial@1493 edb9625f-4e0d-4859-8d74-9fd3b1da38cb