Swift_Message creation is now lazy when using callback

Swift_Message object is not created until right before the message is
sent, if it is sent at all. This prevents the expensive Swift_Mailer
autoloader from being invoked.

Swift_Mailer is now also required to run test suite

tests/Monolog/Handler/SwiftMailerHandlerTest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace Monolog\Handler;
+
+use Monolog\Logger;
+use Monolog\TestCase;
+
+class SwiftMailerHandlerTest extends TestCase
+{
+    /** @var \Swift_Mailer|\PHPUnit_Framework_MockObject_MockObject */
+    private $mailer;
+
+    public function setUp()
+    {
+        $this->mailer = $this
+            ->getMockBuilder('Swift_Mailer')
+            ->disableOriginalConstructor()
+            ->getMock();
+    }
+
+    public function testMessageCreationIsLazyWhenUsingCallback()
+    {
+        $this->mailer->expects($this->never())
+            ->method('send');
+
+        $callback = function () {
+            throw new \RuntimeException('Swift_Message creation callback should not have been called in this test');
+        };
+        $handler = new SwiftMailerHandler($this->mailer, $callback);
+
+        $records = [
+            $this->getRecord(Logger::DEBUG),
+            $this->getRecord(Logger::INFO),
+        ];
+        $handler->handleBatch($records);
+    }
+}