fix(mongo_client_ops): only skip authentication if no authMechanism is specified

Fixes NODE-1840

Author: kvwalker

lib/operations/mongo_client_ops.js

@@ -216,8 +216,8 @@ function connect(mongoClient, url, options, callback) {
       try {
         _finalOptions.credentials = generateCredentials(
           mongoClient,
-          _finalOptions.user,
-          _finalOptions.password,
+          _finalOptions.auth.user,
+          _finalOptions.auth.password,
           _finalOptions
         );
       } catch (err) {
@@ -579,21 +579,14 @@ function transformUrlOptions(_object) {
       object[camelCaseName] = object[name];
     }
   }
-  if (_object.auth) {
-    const auth = _object.auth;
-    for (let i in auth) {
-      if (auth[i]) {
-        object[i] = auth[i];
-      }
-    }
 
-    if (auth.username) {
-      object.auth = auth;
-      object.user = auth.username;
+  if (_object.auth) {
+    object.auth = Object.assign({}, _object.auth);
+    if (object.auth.db) {
+      object.authSource = object.authSource || object.auth.db;
     }
-
-    if (auth.db) {
-      object.authSource = object.authSource || auth.db;
+    if (object.auth.username) {
+      object.auth.user = object.auth.username;
     }
   }