moved the loading of Swiftmailer to the boot method

If not, defining the Swiftmailer class_path after registering the
provider does not work.

src/Silex/Provider/SwiftmailerServiceProvider.php

@@ -79,16 +79,16 @@ class SwiftmailerServiceProvider implements ServiceProviderInterface
         $app['swiftmailer.transport.eventdispatcher'] = $app->share(function () {
             return new \Swift_Events_SimpleEventDispatcher();
         });
+    }
 
+    public function boot(Application $app)
+    {
         if (isset($app['swiftmailer.class_path'])) {
             require_once $app['swiftmailer.class_path'].'/Swift.php';
 
             \Swift::registerAutoload($app['swiftmailer.class_path'].'/../swift_init.php');
         }
-    }
 
-    public function boot(Application $app)
-    {
         $app->finish(function () use ($app) {
             $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']);
         });

tests/Silex/Tests/Provider/SwiftmailerServiceProviderTest.php

@@ -26,6 +26,17 @@ class SwiftmailerServiceProviderTest extends \PHPUnit_Framework_TestCase
         }
     }
 
+    public function testSwiftMailerServiceWhenClassPathIsDefinedLate()
+    {
+        $app = new Application();
+
+        $app->register(new SwiftmailerServiceProvider());
+        $app['swiftmailer.class_path'] = __DIR__.'/../../../../vendor/swiftmailer/swiftmailer/lib/classes';
+        $app->boot();
+
+        $this->assertInstanceOf('Swift_Mailer', $app['mailer']);
+    }
+
     public function testSwiftMailerServiceIsSwiftMailer()
     {
         $app = new Application();
@@ -33,6 +44,7 @@ class SwiftmailerServiceProviderTest extends \PHPUnit_Framework_TestCase
         $app->register(new SwiftmailerServiceProvider(), array(
             'swiftmailer.class_path' => __DIR__.'/../../../../vendor/swiftmailer/swiftmailer/lib/classes',
         ));
+        $app->boot();
 
         $this->assertInstanceOf('Swift_Mailer', $app['mailer']);
     }
@@ -44,6 +56,7 @@ class SwiftmailerServiceProviderTest extends \PHPUnit_Framework_TestCase
         $app->register(new SwiftmailerServiceProvider(), array(
             'swiftmailer.class_path'  => __DIR__.'/../../../../vendor/swiftmailer/swiftmailer/lib/classes',
         ));
+        $app->boot();
 
         $app['swiftmailer.spool'] = $app->share(function () {
             return new SpoolStub();