Serving _site plugins do not pick up on index.html for sub directories

If one asks for `http://es:9200/_plugin/PLUGIN_NAME/` and the the plugin's _site directory contains an index.html file, it will be correctly served.

This is not the case for sub directories: a _site/folder/index.html is not served when requesting  `http://es:9200/_plugin/PLUGIN_NAME/folder/` but one gets a 403 Forbidden response as if trying to browse the folder.

Closes #4845.