[ESS] knitr pdf viewer on windows

[Vitalie Spinu]

Actually the code looks ok. The ess-pdf-viewer-pref should take
effect. Could you please report what is reported by

   M-: (ess-get-pdf-viewer)

Also check that if you call from emacs shell or windows terminal:

start "c:/Program Files/RStudio/bin/sumatra/SumatraPDF.exe" "some_file.pdf"

it really works.

You can inspect what is going wrong with ess-swv-PDF yourself. Go to
that function and instrument with C-u C-M-x. Then call it from rnw
buffer. The debugger will start and you should press n till you reach
the calling part. Then you can inspect all variables and expressions by
executing C-x C-e at the end of those.

 Vitalie