Make URLWrapper.slug settable again
Make URLWrapper.slug settable again

Fixes #2242, Supersedes #2243

getpelican/pelican,ingwinlu/pelican,avaris/pelican,farseerfc/pelican,getpelican/pelican,talha131/pelican,iKevinY/pelican,iKevinY/pelican,ingwinlu/pelican,avaris/pelican,talha131/pelican,farseerfc/pelican