add test for SwiftmailerServiceProvider and update composer.json require-dev section

composer.json

@@ -35,7 +35,8 @@
         "symfony/twig-bridge": "2.1.*",
         "symfony/validator": "2.1.*",
         "twig/twig": ">=1.2.0",
-        "doctrine/dbal": "2.1.*"
+        "doctrine/dbal": "2.1.*",
+        "swiftmailer/swiftmailer": "4.1.*"
     },
     "autoload": {
         "psr-0": { "Silex": "src/" }

tests/Silex/Tests/Provider/SwiftmailerServiceProviderTest.php

+<?php
+
+/*
+ * This file is part of the Silex framework.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * This source file is subject to the MIT license that is bundled
+ * with this source code in the file LICENSE.
+ */
+
+namespace Silex\Tests\Provider;
+
+use Silex\Application;
+use Silex\Provider\SwiftmailerServiceProvider;
+
+class SwiftmailerServiceProviderTest extends \PHPUnit_Framework_TestCase
+{
+    public function setUp()
+    {
+        if (!is_dir(__DIR__.'/../../../../vendor/swiftmailer/swiftmailer/lib')) {
+            $this->markTestSkipped('Swiftmailer submodule was not installed.');
+        }
+    }
+
+    public function testSwiftMailerServiceIsSwiftMailer()
+    {
+        $app = new Application();
+        $app->register(new SwiftmailerServiceProvider(), array(
+            'swiftmailer.class_path'  => __DIR__.'/../../../../vendor/swiftmailer/swiftmailer/lib/classes',
+        ));
+
+        $this->assertInstanceOf('Swift_Mailer', $app['mailer']);
+    }
+}