Only emit vary header for feed if language variants are supported
Only emit vary header for feed if language variants are supported

Change-Id: Iea09e5c0c1882a3f9676045f46faac9f870ff982

milimetric/restbase,Pchelolo/restbase,milimetric/restbase,wikimedia/restbase,wikimedia/mediawiki-services-restbase,wikimedia/restbase,wikimedia/mediawiki-services-restbase,Pchelolo/restbase