[FrameworkBundle] router:debug - better string dumping to avoid namespace escaping

Before this change, the var_export causes namespaces to be escaped:

Before: 'Symfony\\Component\\...'
After:  Symfony\Component\...

src/Symfony/Bundle/FrameworkBundle/Command/RouterDebugCommand.php

@@ -161,6 +161,10 @@ EOF
             return sprintf('object(%s)', get_class($value));
         }
 
+        if (is_string($value)) {
+            return $value;
+        }
+
         return preg_replace("/\n\s*/s", '', var_export($value, true));
     }
 }