Move to strict mode and fix tests

src/Monolog/Handler/NativeMailerHandler.php

@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 /*
  * This file is part of the Monolog package.
@@ -115,7 +115,7 @@ class NativeMailerHandler extends MailHandler
     /**
      * {@inheritdoc}
      */
-    protected function send($content, array $records)
+    protected function send(string $content, array $records)
     {
         $contentType = $this->getContentType() ?: ($this->isHtmlBody($content) ? 'text/html' : 'text/plain');