Move to strict mode and fix tests

tests/Monolog/Formatter/GelfMessageFormatterTest.php

@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 /*
  * This file is part of the Monolog package.
@@ -216,7 +216,7 @@ class GelfMessageFormatterTest extends \PHPUnit_Framework_TestCase
         $length = 200;
 
         foreach ($messageArray as $key => $value) {
-            if (!in_array($key, ['level', 'timestamp'])) {
+            if (!in_array($key, ['level', 'timestamp']) && is_string($value)) {
                 $length += strlen($value);
             }
         }