Send URI to NGINX for Sendfile

Sends URI "{/WEBROOT}/data/{USER}/files/{filename}" to NGINX instead of the full file path.


The [NGINX section of the Sendfile documentation] (http://doc.owncloud.org/server/6.0/admin_manual/configuration/xsendfile.html#id4) should include the changes described [here.](https://github.com/owncloud/core/pull/7838#commitcomment-5752750)

Fixes https://github.com/owncloud/core/issues/7754