Fix bug in createLoggingHandler

Fixes edge-case https://github.com/simplesamlphp/simplesamlphp/issues/741 where application has one of the known LoggingHandlers' name as a defined class

lib/SimpleSAML/Logger.php

@@ -373,7 +373,7 @@ class Logger
             $handler = $config->getString('logging.handler', 'syslog');
         }
 
-        if (class_exists($handler)) {
+        if (!array_key_exists($handler, $known_handlers) && class_exists($handler)) {
             if (!in_array('SimpleSAML\Logger\LoggingHandlerInterface', class_implements($handler), true)) {
                 throw new \Exception("The logging handler '$handler' is invalid.");
             }