fix(db): don't fall back to insert if 'IndexOptionsConflict' error

re: Automattic/mongoose#4459

Author: vkarpov15

lib/db.js

@@ -1046,10 +1046,12 @@ var createIndex = function(self, name, fieldOrSpec, options, callback) {
   createIndexUsingCreateIndexes(self, name, fieldOrSpec, options, function(err, result) {
     if(err == null) return handleCallback(callback, err, result);
 
-    // 67 = 'CannotCreateIndex', means that the server recognized
-    // `createIndex` as a command and so we don't need to fallback to
-    // an insert.
-    if(err.code === 67 || err.code == 11000) {
+    // 67 = 'CannotCreateIndex' (malformed index options)
+    // 85 = 'IndexOptionsConflict' (index already exists with different options)
+    // 11000 = 'DuplicateKey' (couldn't build unique index because of dupes)
+    // These errors mean that the server recognized `createIndex` as a command
+    // and so we don't need to fallback to an insert.
+    if(err.code === 67 || err.code == 11000 || err.code === 85) {
       return handleCallback(callback, err, result);
     }