CachingIterator - Add forgotten constructor parameter. (#4322)

Signed-off-by: Pol Dellaiera <[email protected]>

stubs/CoreGenericClasses.phpstub

@@ -218,7 +218,7 @@ class CachingIterator extends IteratorIterator implements OuterIterator , ArrayA
     /**
      * @param Iterator<TKey, TValue> $iterator
      */
-    public function __construct(Iterator $iterator) {} 
+    public function __construct(Iterator $iterator, int $flags = self::CALL_TOSTRING) {} 
 
     /** @return bool */
     public function hasNext () {}