Fix bad encoding for `tidy_repair_string`

Tidy on PHP 8.5 is more restrictive on what can be given as input encoding. Before, it worked as an unknown value was converted to `utf8`.

src/Readability.php

@@ -118,7 +118,7 @@ class Readability implements LoggerAwareInterface
         'enclose-text' => true,
         'merge-divs' => true,
         // 'merge-spans' => true,
-        'input-encoding' => '????',
+        'input-encoding' => 'utf8',
         'output-encoding' => 'utf8',
         'hide-comments' => true,
     ];

tests/ReadabilityTest.php

@@ -17,6 +17,7 @@ class ReadabilityTest extends \PHPUnit\Framework\TestCase
     public function testConstructDefault()
     {
         $readability = $this->getReadability('');
+        $this->assertSame('utf8', $readability->tidy_config['input-encoding']);
 
         $this->assertNull($readability->url);
         $this->assertInstanceOf(\DOMDocument::class, $readability->dom);