[spec] Str.rindex: Replace comparition to -1 with checking defined() according to spec
[spec] Str.rindex: Replace comparition to -1 with checking defined() according to spec

git-svn-id: 53605643c415f7495558be95614cf14171f1db78@22028 c213334d-75ef-0310-aa23-eaa082d1ae64

dogbert17/roast,b2gills/roast,cygx/roast,dogbert17/roast,zostay/roast,cygx/roast,bitrauser/roast,skids/roast,laben/roast,cygx/roast,laben/roast,dankogai/roast,perl6/roast,skids/roast,niner/roast,niner/roast,b2gills/roast,dankogai/roast,bitrauser/roast,niner/roast,zostay/roast,b2gills/roast,skids/roast,laben/roast,zostay/roast,dankogai/roast