Add ability to use formatter in email subject lines

tests/Monolog/Handler/SwiftMailerHandlerTest.php

@@ -72,6 +72,30 @@ class SwiftMailerHandlerTest extends TestCase
         $handler->handleBatch($records);
     }
 
+    public function testMessageSubjectFormatting()
+    {
+        // Wire Mailer to expect a specific Swift_Message with a customized Subject
+        $messageTemplate = new \Swift_Message();
+        $messageTemplate->setSubject('Alert: %level_name% %message%');
+        $receivedMessage = null;
+
+        $this->mailer->expects($this->once())
+            ->method('send')
+            ->with($this->callback(function ($value) use (&$receivedMessage) {
+                $receivedMessage = $value;
+                return true;
+            }));
+
+        $handler = new SwiftMailerHandler($this->mailer, $messageTemplate);
+
+        $records = array(
+            $this->getRecord(Logger::EMERGENCY),
+        );
+        $handler->handleBatch($records);
+
+        $this->assertEquals('Alert: EMERGENCY test', $receivedMessage->getSubject());
+    }
+
     public function testMessageHaveUniqueId()
     {
         $messageTemplate = \Swift_Message::newInstance();