Fixed some issues in test files

test/groupBy/groupBy.js

@@ -2,5 +2,5 @@ const groupBy = (arr, fn) =>
 arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
 acc[val] = (acc[val] || []).concat(arr[i]);
 return acc;
-}, {
+}, {});
 module.exports = groupBy;