[Ported] fix(connection): default family to undefined rather than 4

lib/connection/connection.js

@@ -46,7 +46,7 @@ var connections = {};
  * @class
  * @param {string} options.host The server host
  * @param {number} options.port The server port
- * @param {number} [options.family=4] Version of IP stack. Defaults to 4.
+ * @param {number} [options.family=null] IP version for DNS lookup, passed down to Node's [`dns.lookup()` function](https://nodejs.org/api/dns.html#dns_dns_lookup_hostname_options_callback). If set to `6`, will only look for ipv6 addresses.
  * @param {boolean} [options.keepAlive=true] TCP Connection keep alive enabled
  * @param {number} [options.keepAliveInitialDelay=300000] Initial delay before TCP keep alive enabled
  * @param {boolean} [options.noDelay=true] TCP Connection no delay
@@ -104,7 +104,7 @@ var Connection = function(messageHandler, options) {
   // Default options
   this.port = options.port || 27017;
   this.host = options.host || 'localhost';
-  this.family = typeof options.family === 'number' ? options.family : 4;
+  this.family = typeof options.family === 'number' ? options.family : void 0;
   this.keepAlive = typeof options.keepAlive === 'boolean' ? options.keepAlive : true;
   this.keepAliveInitialDelay =
     typeof options.keepAliveInitialDelay === 'number' ? options.keepAliveInitialDelay : 300000;
@@ -542,9 +542,15 @@ Connection.prototype.connect = function(_options) {
   }
 
   // Create new connection instance
-  var connection_options = self.domainSocket
-    ? { path: self.host }
-    : { port: self.port, host: self.host, family: self.family };
+  var connection_options;
+  if (self.domainSocket) {
+    connection_options = { path: self.host };
+  } else {
+    connection_options = { port: self.port, host: self.host };
+    if (self.family !== void 0) {
+      connection_options.family = self.family;
+    }
+  }
   self.connection = net.createConnection(connection_options);
 
   // Set the options for the connection