fix function signature tests

Author: xabbuh

src/Util/TestListenerTrait.php

@@ -129,6 +129,8 @@ function {$f['name']}{$f['signature']}
                         'array|string|null $string' => 'array|string $string',
                         'array|string|null $from_encoding = null' => 'array|string|null $from_encoding = null',
                         'array|string|null $from_encoding' => 'array|string $from_encoding',
+                        'string $encoding = null' => '?string $encoding = null',
+                        'int $length = null' => '?int $length = null',
                     ];
 
                     if (strtr($polyfillSignature, $map) !== $originalSignature) {