limit is_html() to only work on the begging of passed string

include/functions.php

@@ -4707,7 +4707,7 @@
 	}
 
 	function is_html($content) {
-		return preg_match("/<html|DOCTYPE html/i", $content) !== 0;
+		return preg_match("/<html|DOCTYPE html/i", substr($content, 0, 20)) !== 0;
 	}
 
 	function url_is_html($url, $login = false, $pass = false) {