Fixed map-with-reduce solution to correctly process array entries

Previous solution was flattening array results of the iterator.

Author: Julian Dreissig

exercises/implement_map_with_reduce/solution/solution.js

@@ -1,5 +1,6 @@
 module.exports = function map(arr, fn, thisArg) {
   return arr.reduce(function(acc, item, index, arr) {
-    return acc.concat(fn.call(thisArg, item, index, arr))
+    acc.push(fn.call(thisArg, item, index, arr))
+    return acc
   }, [])
 }