Add check for cb undefined to prevent calls to ensureIndex with index field and options given but without cb throw an error

lib/collection.js

@@ -183,6 +183,8 @@ Collection.prototype.dropIndex = function(index, cb) {
 Collection.prototype.createIndex = function(index, opts, cb) {
   if (typeof opts === 'function') return this.createIndex(index, {}, opts);
   if (typeof opts === 'undefined') return this.createIndex(index, {}, noop);
+  if (typeof cb === 'undefined') return this.createIndex(index, opts, noop);
+
   opts.name = indexName(index);
   opts.key = index;
   this.runCommand('createIndexes', {indexes: [opts]}, cb);