Resolve #624

Also the testing for isSorted is a lot more robust now.

test/flatten/flatten.js

@@ -1,5 +1,3 @@
 const flatten = (arr, depth = 1) =>
-depth !== 1
-? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flatten(v, depth - 1) : v), [])
-: arr.reduce((a, v) => a.concat(v), []);
+arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);
 module.exports = flatten;