fix(openvpn): do not set tun-ipv6

- Server should push `tun-ipv6` if it is available
- Add ignore filter for `tun-ipv6` if ipv6 is not supported on client
- Fixes #435

internal/provider/utils/openvpn.go

@@ -161,9 +161,8 @@ func OpenVPNConfig(provider OpenVPNProviderSettings,
 		lines.add("persist-key")
 	}
 
-	if ipv6Supported {
-		lines.add("tun-ipv6")
-	} else {
+	if !ipv6Supported {
+		lines.add("pull-filter", "ignore", `"tun-ipv6"`)
 		lines.add("pull-filter", "ignore", `"route-ipv6"`)
 		lines.add("pull-filter", "ignore", `"ifconfig-ipv6"`)
 		lines.addLines(provider.IPv6Lines)