updates-proxy: Disable 'Via: tinyproxy' header

Fixes QubesOS/qubes-issues#1187

network/tinyproxy-updates.conf

@@ -15,7 +15,7 @@ MinSpareServers 2
 MaxSpareServers 10
 StartServers 2
 MaxRequestsPerChild 0
-ViaProxyName "tinyproxy"
+DisableViaHeader Yes
 
 Allow 127.0.0.1
 Allow 10.137.0.0/16