special case 'R-mode' treated during conversion from source to org and from
org to source ('R-mode' returns 'ess-mode' as major-mode, but 'ess-mode' gives
an error when used with 'funcall').
special case `R-mode' treated when wrapping source in source-blocks during
conversion from source to org (use "R" instead of the returned "ess" as
org-babel-language).