DAV authentication: also use Owncloud's internal user for short-circuit
DAV authentication: also use Owncloud's internal user for short-circuit

It still works otherwise, but without this, the performance optimization
of #13416 is defeated in these situations.

andreas-p/nextcloud-server,Ardinis/server,jbicha/server,endsguy/server,bluelml/core,xx621998xx/server,nextcloud/server,endsguy/server,IljaN/core,sharidas/core,nextcloud/server,jbicha/server,cernbox/core,michaelletzgus/nextcloud-server,jbicha/server,nextcloud/server,bluelml/core,endsguy/server,Ardinis/server,michaelletzgus/nextcloud-server,IljaN/core,pmattern/server,owncloud/core,Ardinis/server,owncloud/core,sharidas/core,cernbox/core,sharidas/core,xx621998xx/server,sharidas/core,endsguy/server,IljaN/core,pollopolea/core,pollopolea/core,jbicha/server,michaelletzgus/nextcloud-server,pixelipo/server,pollopolea/core,michaelletzgus/nextcloud-server,pixelipo/server,lrytz/core,IljaN/core,lrytz/core,whitekiba/server,cernbox/core,pmattern/server,whitekiba/server,xx621998xx/server,lrytz/core,pmattern/server,owncloud/core,cernbox/core,andreas-p/nextcloud-server,xx621998xx/server,pixelipo/server,andreas-p/nextcloud-server,pmattern/server,xx621998xx/server,pixelipo/server,IljaN/core,owncloud/core,lrytz/core,whitekiba/server,pmattern/server,pixelipo/server,cernbox/core,andreas-p/nextcloud-server,endsguy/server,Ardinis/server,bluelml/core,owncloud/core,phil-davis/core,pollopolea/core,bluelml/core,lrytz/core,andreas-p/nextcloud-server,sharidas/core,whitekiba/server,nextcloud/server,bluelml/core,whitekiba/server,pollopolea/core,Ardinis/server,jbicha/server