Merge branch '6.1' into 6.2

* 6.1:
  [Translation] Fix Crowdin documentation urls
  ignore missing keys when mapping DateTime objects to uninitialized arrays
  fix writes to static $kernel property
  [Notifier] Fix test logic
  [Validator] Hint that `egulias/email-validator` needs to be installed for strict mode as default config

Tests/FakeSmsEmailTransportTest.php

@@ -96,6 +96,6 @@ public function testSendWithCustomTransport()
         $this->assertSame(sprintf('New SMS on phone number: %s', $phone), $sentEmail->getSubject());
         $this->assertSame($subject, $sentEmail->getTextBody());
         $this->assertTrue($sentEmail->getHeaders()->has('X-Transport'));
-        $this->assertSame($transportName, $sentEmail->getHeaders()->get('X-Transport')->getBodyAsString());
+        $this->assertSame($transportName, $sentEmail->getHeaders()->get('X-Transport')->getBody());
     }
 }