vim: Prevent `set background=dark` from being reset

The call to `<SID>X("Normal", s:foreground, s:background, "")` resets
the background option and since VIm fails to detect the dark background
it gets set to `light`.

Moving the `set background=dark` call at the end fixes it.
Merge pull request #200 from em-/vim

vim: Prevent `set background=dark` from being reset