For each body should be wrapped in an if statement (#14)

unit test to go with commit

src/Phpml/Math/Product.php

@@ -16,8 +16,10 @@ class Product
     {
         $product = 0;
         foreach ($a as $index => $value) {
+            if (is_numeric($value) && is_numeric($b[$index])) {
                 $product += $value * $b[$index];
             }
+        }
 
         return $product;
     }