net: Ensure consistent binding to IPV6 if address is absent

See nodejs#7675
net.server.listen() behaves inconsistently depending on whether the port
number is provided.

1. port === 0 && host == '' (i.e. false-y), node creates an AF_INET
socket but does not call bind().

2. port > 0 && host == '', node creates an AF_INET6 socket and calls
bind().

The fix makes 1 consistent with 2.

lib/net.js

@@ -1091,6 +1091,14 @@ var createServerHandle = exports._createServerHandle =
     } else {
       err = handle.bind(address, port);
     }
+  } else {
+    // Try binding to ipv6 first
+    err = handle.bind6('::', port);
+    if (err) {
+      handle.close();
+      // Fallback to ipv4
+      return createServerHandle('0.0.0.0', port);
+    }
   }
 
   if (err) {

test/simple/test-net-server-address.js

@@ -56,3 +56,45 @@ server_ipv6.listen(common.PORT, localhost_ipv6, function() {
   assert.strictEqual(address_ipv6.family, family_ipv6);
   server_ipv6.close();
 });
+
+// Test without hostname or ip
+var anycast_ipv6 = '::';
+var server = net.createServer();
+
+server.on('error', function(e) {
+  console.log('Error on ip socket: ' + e.toString());
+});
+
+// Specify the port number
+server.listen(common.PORT, function() {
+  var address = server.address();
+  assert.strictEqual(address.address, anycast_ipv6);
+  assert.strictEqual(address.port, common.PORT);
+  assert.strictEqual(address.family, family_ipv6);
+  server.close();
+
+  // Don't specify the port number
+  server.listen(function() {
+    var address = server.address();
+    assert.strictEqual(address.address, anycast_ipv6);
+    assert.strictEqual(address.family, family_ipv6);
+    server.close();
+
+    // Specify a false-y port number
+    server.listen(0, function() {
+      var address = server.address();
+      assert.strictEqual(address.address, anycast_ipv6);
+      assert.strictEqual(address.family, family_ipv6);
+      server.close();
+
+      // Specify -1 as port number
+      server.listen(-1, function() {
+        var address = server.address();
+        assert.strictEqual(address.address, anycast_ipv6);
+        assert.strictEqual(address.family, family_ipv6);
+        server.close();
+      });
+
+    });
+  });
+});