Memoize the return value of Document#url (#6266)
Memoize the return value of Document#url (#6266)

Merge pull request 6266
fulldecent/jekyll,jaybe-jekyll/jekyll,alex-kovac/jekyll,fulldecent/jekyll,ryanshaw/jekyll,x-way/jekyll,Strangehill/jekyll,jekyll/jekyll,tillgrallert/jekyll,yhironaka/yhironaka.github.io,getandpost/jekyll.github.io,chrisfinazzo/jekyll,fulldecent/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,alex-kovac/jekyll,fulldecent/jekyll,gencer/jekyll,tillgrallert/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,x-way/jekyll,x-way/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,jaybe-jekyll/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,gencer/jekyll,ryanshaw/jekyll,gencer/jekyll,alex-kovac/jekyll,x-way/jekyll,Strangehill/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,ryanshaw/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,chrisfinazzo/jekyll,alex-kovac/jekyll,tillgrallert/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,jekyll/jekyll,gencer/jekyll,chrisfinazzo/jekyll