fix; unhandled rejection using Query.then() (Fix #3543)

lib/query.js

@@ -2134,6 +2134,20 @@ Query.prototype.exec = function exec(op, callback) {
   });
 };
 
+/**
+ * Executes the query returning a `Promise` which will be
+ * resolved with either the doc(s) or rejected with the error.
+ *
+ * @param {Function} [resolve]
+ * @param {Function} [reject]
+ * @return {Promise}
+ * @api public
+ */
+
+Query.prototype.then = function(resolve, reject) {
+  return this.exec().then(resolve, reject);
+}
+
 /**
  * Finds the schema for `path`. This is different than
  * calling `schema.path` as it also resolves paths with