Added some tests

Also fixed xProd. Tests for xProd, unzipWith, unzip and is.

snippets/xProd.md

@@ -2,10 +2,10 @@
 
 Creates a new array out of the two supplied by creating each possible pair from the arrays.
 
-Use `Array.map()` to produce every possible pair from the elements of the two arrays.
+Use `Array.reduce()`, `Array.map()` and `Array.concat()` to produce every possible pair from the elements of the two arrays and save them in an array.
 
 ```js
-const xProd = (a, b) => a.map(x => b.map(y => [x, y]));
+const xProd = (a, b) => a.reduce((acc,x) => acc.concat(b.map(y => [x, y])),[]);
 ```
 
 ```js