Avoid double-reg when a Servlet or Filter implements ServletContextInitializer

See gh-4902

spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializerBeans.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -121,7 +121,7 @@ public class ServletContextInitializerBeans
 		}
 		else {
 			addServletContextInitializerBean(ServletContextInitializer.class, beanName,
-					initializer, beanFactory, null);
+					initializer, beanFactory, initializer);
 		}
 	}