Fix handling of encodings in vignettes.  

git-svn-id: https://svn.r-project.org/R/trunk@67165 00db46b3-68df-0310-9c12-caf00c1e9a41
attempted fix for PR15707

git-svn-id: https://svn.r-project.org/R/trunk@67166 00db46b3-68df-0310-9c12-caf00c1e9a41