Setup sendmail transport

Replaces https://github.com/owncloud/core/pull/19047 and fixes https://github.com/owncloud/enterprise/issues/854  and https://github.com/owncloud/core/issues/19110

lib/private/mail/mailer.php

@@ -157,7 +157,10 @@ class Mailer implements IMailer {
 				$this->instance = $this->getSMTPInstance();
 				break;
 			case 'sendmail':
-				$this->instance = $this->getSendMailInstance();
+				// FIXME: Move into the return statement but requires proper testing
+				//       for SMTP and mail as well. Thus not really doable for a
+				//       minor release.
+				$this->instance = \Swift_Mailer::newInstance($this->getSendMailInstance());
 				break;
 			default:
 				$this->instance = $this->getMailInstance();

tests/lib/mail/mailer.php

@@ -77,7 +77,7 @@ class MailerTest extends TestCase {
 			->method('getSystemValue')
 			->will($this->returnValue('sendmail'));
 
-		$this->assertInstanceOf('\Swift_SendmailTransport', self::invokePrivate($this->mailer, 'getInstance'));
+		$this->assertInstanceOf('\Swift_Mailer', self::invokePrivate($this->mailer, 'getInstance'));
 	}
 
 	public function testCreateMessage() {