Add json extension to list of directory indices (#6550)
Add json extension to list of directory indices (#6550)

Merge pull request 6550
tillgrallert/jekyll,ryanshaw/jekyll,fulldecent/jekyll,jekyll/jekyll,ryanshaw/jekyll,x-way/jekyll,x-way/jekyll,Strangehill/jekyll,Strangehill/jekyll,gencer/jekyll,x-way/jekyll,ryanshaw/jekyll,Strangehill/jekyll,fulldecent/jekyll,x-way/jekyll,tillgrallert/jekyll,yhironaka/yhironaka.github.io,getandpost/jekyll.github.io,jaybe-jekyll/jekyll,Strangehill/jekyll,gencer/jekyll,fulldecent/jekyll,jaybe-jekyll/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,Strangehill/jekyll,gencer/jekyll,jekyll/jekyll,jekyll/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,getandpost/jekyll.github.io,gencer/jekyll,tillgrallert/jekyll,fulldecent/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,getandpost/jekyll.github.io