network: detect default ipv6 interface when no ipv4 interface

src/test/network-test.js

@@ -141,4 +141,10 @@ describe('Network', function () {
             expect(await network.getDynamicDns()).to.be(true);
         });
     });
+
+    describe('Default interface', function () {
+        it('can get default interface', async function () {
+            expect(await network.getDefaultInterface()).to.be.ok();
+        });
+    });
 });