Made the write method protected

Tests are not a good reason to make it public.

tests/bootstrap.php

@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Monolog/TestCase.php';
+
 spl_autoload_register(function($class)
 {
     $file = __DIR__.'/../src/'.strtr($class, '\\', '/').'.php';