MC-33700: TransportBuilder unable to send emails of Content-Type "text/plain"

Author: Roman Lytvynenko

dev/tests/integration/testsuite/Magento/Framework/Mail/TransportBuilderTest.php

@@ -81,7 +81,7 @@ public function testAddToEmail($email)
         /** @var EmailMessage $emailMessage */
         $emailMessage = $this->builder->getTransport()->getMessage();
 
-        $this->assertContains($templateType, $emailMessage->getHeaders()['Content-Type']);
+        $this->assertStringContainsStringIgnoringCase($templateType, $emailMessage->getHeaders()['Content-Type']);
 
         $addresses = $emailMessage->getTo();