Fixes tag replacement in wcf\system\MetaTagHandler

cf. #1198

If you try to replace a tag, a new index is added to the indexToObject array, which later returns the same tag (same identifier!) twice during iteration.

Test case:

	<?php
	require_once('./global.php');    
	@header("Content-type: text/plain");
	
	$tagHandler = wcf\system\MetaTagHandler::getInstance();
	
	foreach($tagHandler as $tag)
		echo $tag.PHP_EOL;
	
	echo '+++' . PHP_EOL;
	
	$tagHandler->addTag('description', 'description', 'fuuu!');
	
	foreach($tagHandler as $tag)
		echo $tag.PHP_EOL;

Output:

	<meta name="description" content="" />
	<meta name="keywords" content="" />
	<meta property="og:site_name" content="" />
	+++
	<meta name="description" content="fuuu!" />
	<meta name="keywords" content="" />
	<meta property="og:site_name" content="" />
	<meta name="description" content="fuuu!" />

It should be clear, that if you replace the tag with the same identifier, it should not be displayed twice. The proposed pull request fixes the issue by only creating a new index in the indexToObject array, if and only if the identifier was not used before.