[t/spec] Add test that .slurp returns a Str.
[t/spec] Add test that .slurp returns a Str.

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