Change timezone to be per-instance and not global

tests/Monolog/LoggerTest.php

@@ -491,8 +491,8 @@ class LoggerTest extends \PHPUnit_Framework_TestCase
      */
     public function testSetTimezone($tz)
     {
-        Logger::setTimezone($tz);
         $logger = new Logger('foo');
+        $logger->setTimezone($tz);
         $handler = new TestHandler;
         $logger->pushHandler($handler);
         $logger->info('test');