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

unit test to go with commit

src/Phpml/Math/Product.php

@@ -16,7 +16,9 @@ class Product
     {
         $product = 0;
         foreach ($a as $index => $value) {
-            $product += $value * $b[$index];
+            if (is_numeric($value) && is_numeric($b[$index])) {
+                $product += $value * $b[$index];
+            }
         }
 
         return $product;